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Psychrometric Chart help with Air conditioning.
3

Psychrometric Chart help with Air conditioning.

Psychrometric Chart help with Air conditioning.

(OP)
This may sound like a silly question but I have totally forgotten how to use the Psyh- chart to follow a air conditioning system problem.

For example if I start with an outdoor condition of 95 degrees F and 80% Relative Humidity and I mix that air with return air that is at 80 degrees F and 50% relative humidity.  The mix is 80% Return air and 20% supply air.

I then take that mixed air and run it thru a evaporator to cool it down to a disscharge temperature of 36 degrees F.

My question is how to properly do this but more importantly how to graph this on a Psychrometric Chart.  I am confused as to how the lines would appear.  I thought that in cooling you went horizontally from right to left on the chart and then followed the due pointline down to your disscharge temperature.  As you did that you would lose water at the evaporator.  Then when you got to the line associated with 36 degrees F at 100% realtive humidity you stoped going down and then moved horizontally to the right on the chart until you readed your room tempererature which in my case is 80 degrees F.  But I don't knwo if that is the right way to do it or not.

I am confused right now and need soem explanation.

RE: Psychrometric Chart help with Air conditioning.

There is an article on psychrometrics in the July 2002 edition of ASHRAE Journal.  If you are an ASHRAE member or know one, I think you can still download it.  Otherwise you might be able to get it at a university library or through inter-library loan.  Alternatively, there is probably something in the ASHRAE Handbook of Fundamentals which you could get at a library or buy an older used copy off Ebay.  Someone may be able to explain well with words but seeing it done on the chart would be much easier to understand.  

RE: Psychrometric Chart help with Air conditioning.

(OP)
My problem is I need this information yesterday.  LOL

I am working on an issue that I need to solve before the end of the day today and I need quick explanation.

RE: Psychrometric Chart help with Air conditioning.

Your room condition is the return air.  To find your mixed air, plot both the return and outside points.  The mixed air will be 20% of the distance "up" the line.  In other words, it will be 83 deg and roughly 60% RH.

When leaving the coil, you will move both to the right and up to get back to your room codition (return air), as you have both a sensible and latent load.  

RE: Psychrometric Chart help with Air conditioning.

3
Jasno99,
  Here are some rough numbers assuming sealevel 29.92" Hg. Your mixed air sould be about:
82 degrees Db; 71 degrees Wb; 58% Rh
35 degree supply air off the coil is pretty cool for standard comfort cooling maybe this is a special application.
At 35 degrees 95% Rh (few coils can produce 100% Rh)
11.0 Btu/Lbs dry air sensible heat
10.2 BTU/Lbs dry air latent heat
21.2 BTU/Lbs dry air Total heat
.518 sensible heat ratio
Moisture removal
.0093 Lbs water/Lbs dry air
Specific Volume @ 35 degrees
12.56 Cu.Ft./Lbs dry air

  That should cover it there are some pretty good site on the net that go over the basics. Hope this helps

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

I need to appologize I made an error in plotting the OSA the corrections are as follows
Mixed air: 83 degrees Db; 72.5 degrees Wb; 60% Rh
36 degree supply air off the coil is pretty cool for standard comfort cooling maybe this is a special application.
At 36 degrees 95% Rh (few coils can produce 100% Rh)
11.4 Btu/Lbs dry air sensible heat
11.3 BTU/Lbs dry air latent heat
22.7 BTU/Lbs dry air Total heat (this is load seen by the  coil
.502 sensible heat ratio
Moisture removal
.0104 Lbs water/Lbs dry air
Specific Volume @ 36 degrees
12.56 Cu.Ft./Lbs dry air
  I'm very sorry for the error hope that hasn't caused you too much trouble

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok I guess the question then is this:

How do I figure out what the required coolign capacity is for the system.

I know that I find a structural heat gain, electrical heat gain, human loading and maybe infiltration air if we have some.  I can add those up and figure out what the heat gain in the room is.  But how do then figure out the nessassary cooling required to maintain that room at say 80F?

Obviously I need to provide enough cooling to make up for the heat gain we see in the room.  But I assume that I amso need to provide coolign to make up for the humidity in the outdoor air (or mixed air entering the evaporator)???  I am not sure how to figure out the total system loading.

RE: Psychrometric Chart help with Air conditioning.

Last night time was short to explain thouroughly.
 This assumes your using an ASHRAE sealevel chart

1.Plot OSA point on chart at 95 degrees 90% Rh

2.Plot RA point on chart at 80 degrees 50% Rh

3.Connect the OSA point and the RA points for mixed air line.

4.Measure length of line 3.875" x 20% = .775"(there are other methods to find this and I can give a simple formula if you need one let me know.)

5.Measure along the MA line starting from the RA point .775" this is your MA temperature and humidity.Read 83 degrees 60% Rh

6.Plot the supply air point (SA) 36 degrees 100% Rh (depends on the efficiency of the cooling coil) I used 95%

7.Connect straight line from the MA point to the SA point

8.Form a right triangle using the SA point and the MA point,making a horizontal line fom the SA point and intersecting it with a vertical line from the MA point.

9. Locate enthalpy values at the SA point, the MA point, and at the 90 degree intersection of the horizotal and vertical lines of the triangle. These enthalpy lines run diagonally care must be used in accuratly locating.

10. At SA point read 13.4 BTU/Lbs, at MA point read
36 BTU/Lbs, at 90 degre intersection read 24.8 BTU/Lbs

11.The difference between 24.8 and 13.4 is sensible heat BTUs, the difference between 36 and 24.8 is latent heat BTUs, the total heat is the difference between 36 and 13.4 This is the load at the cooling coil, not the load in the room. The load in the room would be figured from the RA point.

12.To find the LBS of water removed extend horizotal lines across chart from the SA point and the MA point to the humidity ratio index at far right of the chart. For the SA point read .0042LBS+/- and the MA point read .0104+/- the difference is the LBS of water removed per Lbs of dry air .

 Hope that wasn't too detailed point acrossed. Good luck

PS. ASHRAE sells a real nice psychrometris program for about 150.00 USD  
 

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

Item 12 should read

12.To find the LBS of water removed extend horizotal lines across chart from the SA point and the MA point to the humidity ratio index at far right of the chart. For the SA point read .0042LBS+/- and the MA point read .0146+/- the difference is the LBS of water removed per Lbs of dry air. Aprx. .0104 Lbs of water /Lbs of dry air
 Sorry way too many numbers for 6:00 a.m.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

One last point, you need to determine the CFM based on the room load and required outside air. Once you know the CFM, you can use the formula
BTU/HR = 4.5 x CFM x ΔH
to find the total cooling coil load

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Yorkman - Wow that was very helpfull.  I have to now go use your explanation on a few of the issues I currently have.

Right nwo I am tryign to figure out if a quote I have for a project has the correct numbers in it.  They gave me the room BTUH but I need to go back and see if that number is right and then I need to see what the real heat load of the evaporator is to see if they sized their condenser jsut right.  I appreciate the help.

Now again where I am seeign an issue is you gave me the equation BTU/HR = 4.5 x CFM x ?H

But I think the real equation should read BTU/HR = 4.5 x CFM x ?T X 0.24  (I thought this was a delta Temperature not an enthalpy).

But what I was tryign to do was look at the sensiable heat and latent heat using equations to find the total heal load at the evaporator.

I was using the BTU/HR = 4.5 x CFM x ?T X 0.24 for my sensiable load and I was using the equation 0.7 X CFM X ?HR(grains/lb.)as my latent load equation.  But I think this is nto the right way to do it for some reason.  I was using the Mixed air properties and the Supply air properties to fill in the ?T and ?HR numbers.

RE: Psychrometric Chart help with Air conditioning.

(OP)
How about a situation where I am given the following:

I know the OA conditions,  I know the internal Temperature requirment,  I am given a capacity of the system, and I have some data for what the manufacturer says the internal humidity can be maintained at.  I also knwo the fan flow of the system.  Last but not least I knwo what the structural heat loads are, human healt loads are and electrical loads are.  I am still running a 80% return air system.

How do I go about figering out what the supply temperature would be given that information?  How would I go about determinng if the loads given to me by the manufacturer were good numbers and would be able to handle the conditions and give me the temperature requirment in the room?

RE: Psychrometric Chart help with Air conditioning.

I haven't read everyone's reply, but here are my 2 cents:

The coil load will be the difference between the mixed air conditions and the coil leaving air conditions.  In your example, the mixed air should be roughly 83 deg and 60%RH.  The mixed air enthalpy (h) is around 36 but/lb, and you coil enthalpy is around 13 btu/lb.

Q (total)=(4.5)*(cfm)*(delta-h)

you can break up sensible and latent if you wish:

Q (sensible)=(1.08)*(cfm)*(delta-T), where T is temp diff
Q (latent)=(0.7)*(cfm)*(delta-HR), where HR is humidity ratio in grains/lb

Either way, if you know cfm, you can find Q.  

If you are having trouble finding the mixed air condition, here is an easy way.  Plot both on the psyc chart and draw a straight line connecting them.  Then use simple math the determine the mixed air temp:

Mixed T = (80 deg)*(80%)+ (95 deg)*(20%) = 83 deg

Go to 83 deg on the line you drew and find the corresponding RH and other properties.  This is your mixed air point.

RE: Psychrometric Chart help with Air conditioning.

(OP)
BronYrAur - I have been readign the posts but my last question was a little different because I was searching for the disscharge temperature off the coil with a different set of given conditions.  It is basically a different question from the first one I asked.

I think that Yorkman did a great job explaining that first example and showing me how to look at things.  YOu sort of restated it and showed me the breakdown for the sensiable verses the latent.  When I have done thee calculations before with the breakdown I have notieced that my total for sensiable and laten typically does nto equal my total for enthalpy alone.  They are close but not the same.  Maybe my numbers are jsut off rome not being perfect when readign the psyh chart.  I can double check tomorrow.

EIther way I still appreciate you help but and looking for answeres to a sort of different question.

RE: Psychrometric Chart help with Air conditioning.

Hey Jasno999,
  Just got home from work, lets look at what you have.
BTU/Hr = 4.5 x CFM x ΔH is a total heat formula for air at sealevel. It does not accept ΔT's in this form, but can be adjusted the this one in my opinion is easier. You can use it to find required CFM,or total BTU/Hr depending on the data you have, provided you know the enthalpy values of your supply and return/space air.
 BTU/Hr = 1.08 x CFM x (T1-T2) is used for sensible heat problems generally.
  To find the answers you are looking for requires the use of the sensible heat ratio. The sensible heat ratio is a ratio of sensible heat to total heat based on your space load. The protractor on the upper left side of the psychrometric chart lets you plot the ratio or slope on the chart. Look closely at the protractor, the numbers on the inside radius are the ratios of sensible heat. Suppose your sensible heat ratio for your problem was .60 based on your rooms' sensible and latent heat load

1.Draw a line from the center point on the base line of the protractor through the .6 on the radius of the protractor. This establishes your slope.

2.Now plot your desired Db and %RH for the space on the chart, 80 degrees Db 50%RH.  

3.Transfer the sensible heat line or slope (remember the protractor?) so that it starts at your desired room temperature and travels to the left and intersects the 90% curve on the chart at 90%Rh (read 45 degrees Db).

4.The point of intersection will establish the required supply air temperature to maintain the desired space temperature.  

5.You could select any desired supply air temperature located on this line in theory and adjust your CFM, but most A/C equipment delivers air at 90% Rh or higher so  you are some what limited, so much for theories.

6.Once you have established your supply air temperature, use the enthalpy values for the supply air and desired space condition and enter them into the :
BTU/Hr = 4.5 x CFM x ΔH formula along with your total room load in BTU/Hr to find the required CFM.

If that don't make your head hurt I don't know what will!!
I might suggest a good book on the subject by The Trane Company "Trane Air Conditioning Manual" excellent resource for this type of work as well as just all around good A/C therory. As I posted before ASHRAE has a very good and affordable "Psychrometric Analysis" program.
  This was fun hoped it helped, that is about the limit of my skills without spending a couple of nights of exciting reading dazed Good luck!!

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

After reading your post with the formula
"But I think the real equation should read :
BTU/HR = 4.5 x CFM x ΔT X 0.24  (I thought this was a delta Temperature not an enthalpy)".

Is correct, but:This might help in understanding the theory.

 BTU/HR = 4.5 x CFM x ΔT X 0.24 will work, but it is only for sensible heat applications)
          Note the breakdown:
.075 (density of air) x 60 min./Hr = 4.5
4.5 x .24(specific heat of air, BTU to change 1 Lbs of air 1 degree F) = 1.08  (Lbs/Cu.Ft./Hr/degree change) ouch!

When using: BTU/Hr = 4.5 x CFM x ?H  keeps the units in Lbs/Cu.Ft./Hr while allowing us to use Enthalpy BTU/Lb. A value not limited to only sensible heat changes it can also represents latent heat changes or both.. Hope that makes sense.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

Jasno,

Your latent heat formula is also approximately correct. The latent heat is due to moisture gain and if you want to express the latent heat as a multiple of cfm and moisture gain,

Q = k x cfm x gr/lb

If you expand the equation for dimensional stability,

Q (btu/hr) = k*cu.ft/min*(60min/hr)*(gr/lb)*(7000/7000)*(1000btu/lb)* (0.075lb/cu.ft)

upon simplification and rearranging the terms,
Q (btu/hr) = k*60*0.075*1000/7000 (btu/hr)
or k = 0.64 (which is close to your constant)

I would like to explain one part of Yorkman's excellent post. The SHR line cutting at 90% curve is an indication of coil bypass factor (BPF). When you pass the mixed air through the cooling coil some portion of it gets bypassed (with better coil fin arrangement, a safe bet can be about 5%). So, 5% is untreated mixed air and 95% is treated mixed air. This point can be marked on the SHR line (drawn from the room condition), in the same way as explained for mixed condition.

Actually coil ADP is identified after you do this exercise. I am puzzled as to how you could get it well in advance.

Sometimes, if the latent heat portion is high, the SHR line may not intersect the saturation line and runs almost parallel to it. In this case, you have to assume a coil ADP (below room entry point)and include reheat into the system. The entry point is calculated by assuming some DB temperature difference (say, 15 to 20F) from room entry point and room condition.

Yorkman,

Excellent work (and energy - in physiological sensewink). For quite sometime, I have been thinking of writing a FAQ in HVAC forum about this but couldn't do it due to laziness. We all will be greatful if you can you convert this excellent post into a FAQ.

Best regards,


RE: Psychrometric Chart help with Air conditioning.

Quark,
    Thanks for the personel comments and the added explaination on SHR, by the way what is ADP? blushing I' not familiar with the acronim please expand. I've been chided by some for using a psychrometric chart for analysis rather the a packaged program. While I think the programs are very valuable and fast, I still enjoy the act of plotting out the points and crunching the numbers, although I fear the process may be going the way of the slide rule. sad winky smile    
  One of the reasons I enjoy this site so much is the information gained is always more than what one individauls' experience could provide. I've never posted a FAQ and the request means alot. I'm leaving for a long weekend but when I get back I will do my best to assemble some of the more coherent thoughts, into a quick primer on using the psychrometric chart. I may touch base with you on a few points, Thanks

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok this is what I did. (NEW EXAMPLE)

I looked at my structural losses (at the given outdoor conditions of say 105 & 60%RH) thru the walls & ceiling of the room.  I then added that to my electrical heat gains in the room.  I then added my sensiable heat gains from people in the room who would be working with a room temperature of 80F (I used charts to figure that out with the proper equation).  And finally I added a 10% leakage loss factor on to the entire equation.

I took all of those values and added them together.  For this new example those values came out to be 10,000 BTUH.  That should be the total heat gain the room sees so we need to make sure the cooling system is able to cool the air enough to equal this amoutn of heat gain.


I was also told that my room conditions would be maintained at 80F and 50%RH.  Knowing that I figured I could use the sensicabel equation to find out what the temperature of the air was enetering the room.

SO Qs = Flow X 60 X 0.24 X Delta T

Qs = 10,000 BTUH
Flow = 37.5 lb/min
Indoor air temp = 80F

Therfore:

Tsa = 80F - (10,000/(37.5 X 60 * 0.24))

Tsa = 61.5F

IS THAT THE RIGHT WAY TO DO THAT?  OR DO I NEED SOME TYPE OF LATENT HEAT IN THIS EQUATION FOR THE ROOM?  I figured that since moisture is nto being removed between the supply air and the room that latent did nto factor in here.

I figured this was all sensiable and that the latent factored in where moisture was being removed and that woudl be at the evaporator where you would need to look at bout the sensiabel and the latent loads.

I could be wrong but that is the question.  Did I figure out the supply air temperature Tsa correctly?

If I had the supply air temperature I sould then assuem a 95% RH at the supply air out of the evaporator and then used the mixed air conditions prior to the evaporator along wit hthe supply air conditions to determine the heat rejection required at the evaporator.

RE: Psychrometric Chart help with Air conditioning.

The latent load of the people and from infiltration must be factored in as load in the room. Take the 10,000 BTU/HR sensible plus the latent BTU/Hr from infiltration and humans added together to get total heat. Find the SHR based on these totals. Plot desired room Db and Wb temps draw SHR line to 95% curve on psychrometric chart this will be required supply air temp. Then use Qs = Flow X 60 X 0.24 X Delta T where you know Qs; Supply air and room air to solve for CFM, you shouldn't choose the CFM you need to calculate it
Next use :  BTU/Hr = 4.5 x CFM x ?H where you know the required  CFM that you just found, the enthalpy of the mixed air, and the supply air enthalpy to find the coil capacity need for the system load. Thats how I would approach it.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
What I have is a vendor that has selected a system for me and given me a quote.  He has speced out all of the thigns liek the flow rate, the system overal BTUH and he has given me the room temperature and humidities that he says are possiable given specific outdoor air conditions.

So what Iam tryign to do is figure out if what he gave me works.  So I am using the formulas to basically find the supply air temperature and humidity.  

However since I know that there is moisture being added to the air via the people in the room I need to factor that into everyhting.  

So if I get a guy that tells me the best indoor air condition he can get me when it is 100F and 80%RH outside is an indoor temperature of 80F @ 90%RH then I need to figure out what he is saying.  I need to try to figure out what the supply air temperature is.

See if moisture was not being added then you could eaisly find the disscharge air temperature cause it should basically be a horizontal like from the end point (or room condition) right???  But when moisture is added you don't jsut go horizontally on the chart but you also go up.  So you end up with a diagnoal line on the psyh chart.  What I need to do is determine the length of that line or better yet determine the amount of moisture added lbw/lba so that I can figure out what the supply temperature really is.

It soudns confusing but I don't know how better to explain it.

I know that my supply air will be at aroudn a 95% RH.  But I don't knwo what it is cause I need to look at the moisture added to the room and then go diagnally from the end condition of 80F @ 90%RH to the supply temp @ 95%RH....
In order to do that I need to figure out the actual moisture added and subtract it out.  I jsut don;tknow how to do that.  I know how to find my moisture added in lb/min but I do nto know how to relate that to the chart to find my moisture addition.

RE: Psychrometric Chart help with Air conditioning.

(OP)
For example let me simplify some.

When lookign at the psyh chart if you want to cool and dehumidify the following should occur.  Please let me know if I am incorect in my thought process.

YOu start at a temperature and humidity.  As you cool you move horizontally to the left on the chart until you hit the dew point line.  The cooling doen in this phase is sensiable and deals on with cooling of the air temperature.

Now you have hit the dew point line you will start to ride the dew point line down and to the left until you reash your disscharge temperature out of your evaporator.

At that point you will have a disscharge temperature at around 95%RH coming out of the evaporator.   This air is then heated back up as it enters the room and mixes with the room air.  However since you are not addin any moisture in this part of the process the movment shoulw be on the horizontal line from left to righ across the chart, starting at your disscharge air temperature and humidity.   You then go right until you hit the room temperature that is desired (Vertical line) and that is point at wich you can then see what your room humidity is.

Therfore if a supplier tells me that they can maintan the room temperature and humidity at 76F and 80%RH then that means to me that their supply air temperature must be around 69F.  

But I think that I am missign somethign here. It confuses me - because if that is what I am told the room air conditions will be then I have to make those assumptions.  But that makes little to no sense to me cause it means the disscharge air temperature is extreamly warm...

RE: Psychrometric Chart help with Air conditioning.

(OP)
I think I came to a conclusion and that is I am viewing the problem wrong.  In reality when supply air coming from an evaporator enters a room you actually have a mixing process going on right?

Again I am not sure how to figure out the supply air temperature and humidity content.

If I know the following:
- Overall Room Temperature
- Overall Room Humidity
- Outdoor air temperature
- Outdoor air humidity
- Evaporator fan flow rate (lb./hr)
- Room Structural Load
- Room Human Load
- Room Solar Load
- Room electrical Load
- Room Infiltration Load

Then how do I figure out what the required supply air temperature and humidity are out of the evaporator?

I sort of understand the SHR.  However you need to know your total heat load along with how much of it is related to latent load and sensiable load.

Now I am guessign that the total heat load would be the addition of the structural, electric, solar, human and infiltration loads.  IS THAT CORRECT?

Now how would I be able to figure out what portion of that load is sensiable and what portion is latent?  

If you can't do that without knowedage of the supply air temperature to begin with then is there another method to solve for the supply air temperature and humidity.

I actualyl own the Trane Air Conditioning Manual.  I have been readign it but I am still confused.  I have been lookign at your explanations and tryign to make them work as well.  But the problem is no matter what I do it is liek I am missing a variable or the equations I have don't work or have a factor missign cause I don't know the supply air temperature and humidity.

I understand that the supply air to room air is a mixing process really but I just can't figure out how to work it all out.  I have seriously been rackign my brain for the past week and a half tryign to figure this out.  I have reached a lot of dead ends.  I need soem more help before my brain goes wacky on me from frustration.

RE: Psychrometric Chart help with Air conditioning.

(OP)
Note my humidities listed above that are known are measured in %RH

RE: Psychrometric Chart help with Air conditioning.

Jasno999,
  Sorry that I haven't been responding been out of town. Go to Trane Air conditioning Manual: Chapter V
Example 5-26; Pg 99 that is a pretty close representation of your problem if I understand it. The sensible heat loads are structural loads, equipment loads, some %infiltration load, and some %occupant load. The latent loads are moisture generated by cooking, washing, coffee pots, a % of the infiltration load and a % of the occupant load. (An ASHRAE Fundamentals manual can be helpful in determining these values). These two loads combined is your total heat load in BTU/Hr for the space. Take the sensible heat load in BTU/Hr and divide it by the total heat load to get your SHR. Use the sensible heat protractor and generate the SHR slope for the space load. Plot the desired Db temperature and % Rh for the space. Transfer the SHR line from the desired space temperature point to the 95% Rh curve on the chart. The intersection point of the 95% Rh and the SHR line will give you the required supply air Db temperature @ 95% Rh. Use the formula
BTU/Hr = 1.08 x CFM x (T1-T2) with the known sensible heat load for BTU/HR, T1 as the space temperture and T2 as supply temperature, you can now solve for the CFM.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

I've posted this as a FAQ in the HVAC section I don't guarrentee it will make any more sense over there, but it may help.dazed

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
No I really appreciate the help Yorkman.  I have only been out of college for 6 years not but when you never use something for a period of time that long you foget stuff.  And truthfully I am not sure how many realy world problems we did in college.

I think I understand what you are saying so today I wil lcrack the book back open and try to resolve this problem and see what I come up with.  Again I am sort of doign this backwards because I am not really tryign to design the system I am just tryign to make sure that the design given to me will work.  But I guess the best hting I could do is pick some points that I woudl liek to get to and then solve the equations and find the CFM required and compare it against what I am receiveing from my vendors.


I think it was a simple point that I was missing for the SHR.  So now that you explained that to me I think I can use it.  I still do nto fully understand it however.  I knwo that as you add cold wet air to a room a few thigns happen.  As that air heats up it becomes less humid.  However at the same time you are mixing with room air that has a given temperature and humidity.  YOu are also adding moisture via occupant sweat and infiltration.  But my latent loading from the infiltration and occupants is only goign to be a small portion of the total heat load.  

Let me work the problem and see what I come up with.

I may have some follow up questions about the evaporator loading after I get this portion figured out.  You had helped me soem with that part already so I will go back to your previous posts but if I get stuck I will post some more questions.

Again I really appreciate it.  YOur help is really fantastic.  Wish there were more people like yourself out there. (This whole thing has got me thinking maybe I should take a class at one of the local colleges to refresh and learn more about the equations behind HVAC)

RE: Psychrometric Chart help with Air conditioning.

Jasno99,
 I think you got it, the SHR line in reality shows the mixing process of the supply air with the rooms latent  and sensible heat loads to arrive at the desired room condition. Because you used the rooms sensible and latent heat loads to find the SHR line, when you select your desired room condition and use the SHR line to find the required supply air condition, it should be a perfect match. The problem can get a bit more involved when the SHR line is steep (reflecting a high latent load in the room) because it does'nt intersect the 90-95%Rh line making the supply air unreasonably cold and usually requires some form of reheat. I believe Quark mentioned that point earlier in the thread take a look. Also check out the FAQ in the HVAC forum I posted a small point on finding the load on the coil due to mixed air condition. Goodluck

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok here is a good question.  

If you are solving for your CFM supply rate how do you find items such as your infiltration latent and sensiable loading and how do you get your latent human loading.

see those equations can only be used if you know the following:

- % of recirculation air
- Infiltration of outdoor air flow rate (CFM)
- Humidity Ratio (grains/lb) of indoor air

Are you supposed to make assumptions for thee items?  You need to know them so that you can get to the answers but it seems like each equation is based on or depends on the answer to other equations.  So how do you figure it all out if all you know is your structural loads, electrical loads, solar loads but not your flow rates or internal conditions?

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok I used the formulas and tried to figure the whole thing out.

What I did was take several outdoor air conditions and I made some assumptions.  For example I assumed a % for return air.  I assumed a relative humidity of 60% for the room and know I needed to maintain that with an 80F temperature.  I aso made an assumption for leakage.

What I found was that different conditions provided different loading requirments.  Makes sense.  The other thing that I found was that as ,y temperature increased so did the flow requirment that I was calculating.

Now here is the question.  You said you need to solve for CFM but my CFM is different for each different condition.  So in the real world I would think that you would select one CFM number (probabbly a lower CFM if possiable) and then use that for all the conditions.  

What that meas is you set your CFM for all conditions and instead of solving for it you then need to use it to solve for your supply air disscharge temperature.  Is that correct???

I see no other way of doing it but if you do it this way then you are throwing out the whole idea of the SHR and how that works on the psyh chart...

I have a little confusing in my mind about this.

RE: Psychrometric Chart help with Air conditioning.

Yes that is correct. The SHR is calculated at design load. Using this to find the design CFM, that means that it will meet the max. "reasonable" load your going to see. Some days you will be at design load, some days you will be less than design load and every once in a while your load will be over design. That's why G-D made thermostats! Most of the time you won't be running at design load in a comfort cooling application. Designing the system to meet a 100% design criteria means your paying for equipment that won't be run at that level, what maybe 70% of the time? The trick is finding a balance that wiil meet the load 97-99% of the time and still being able to justify the cost. That's why you guys get paid the big bucks!
  By the way where are you located? A room design temperature of 80 degrees and 60 %Rh seems a bit high. Most customers I know start screaming when the space temp hits 76-77 degrees 45-50% Rh!   

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ha ha.  I used to work for an HVAC company but never did loading calculation.  I was a controls person.  So I know that side of the buisness but never got involved with the actual engineerign side.

The project I am talking about is unique and it is not a comfort cooling application.  I do realize that 80F would be hot for people in an office building - trust me when you are in controls the majority of your complaints are for people being too ho or too cold.  But I am sure you know that.

Again I appreciate the help.  I think I am on the right track now.  I at least feel a lot more confident about my calculations and the numbers I am coming up with.  I am sure a few more questions might pop up and when they do I will post them - but again your help has been tremendous.

Thanks

RE: Psychrometric Chart help with Air conditioning.

(OP)
HEre is another interesting question for you.

One thing I noticed is that manufactures will give you two different values for heat transfer.  

They give you the condenser heat rejection value (BTU/hour) and they also give you the system cooling value in (BTU/hour).

The numbers that I am calculating for the evaporator Q - are they supposed to correspond with the overall system cooling load or do they correspond with the load of heat rejection at the condenser?

The heat transfer rates at the condenser are larger than those given for the overall system cooling in my documents.  I need to know what to compare the number I am calculating against.

RE: Psychrometric Chart help with Air conditioning.

Glad I could help,
 If you post these type of questions (HVAC) in the HVAC section you may get better attention, and a faster response.  

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

The condensers' rate of heat rejection has to be higher than the system cooling load. Remember that for every 12,000 BTU/Hr picked up in the evaporator the condenser must reject about 15,000 BTU/Hr (or more on hermetic compressors) due to the heat of compression. This is the price of doing work on the gas. When figuring system load I would go with the evaporator Q values, with a package unit the condenser is sized to reject the additional heat of compression. On a built up system were you are selecting the condenser a quick discussion with the supplier should lead you to the correct condenser size, it will probably reject 15,000-17,000 BTU/Hr for every 12,000BTU/Hr picked up in the evap.  

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

Jasno999,

I hope, by this time, you got answers to all your question by well articulated suggestions from Yorkman. Regarding cfm, you have to choose the maximum cfm you have got out of various requirements.

We in Pharma, know the cfm well in advance before starting any psychrometric calculations as the no. of air changes per hour is critical.

Yorkman,

Excellent work. The ADP is apparatus dew point (generally referred to as dew point).

To all,

I sincerely feel that the efforts and knowledge of yorkman should not go unappreciated. We have stars handy with us, why wait?

RE: Psychrometric Chart help with Air conditioning.

Quark, Thanks.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

By the way, does anybody knows about available dynamic Psychrometric Chart program?

Thanks in advance.
welch  

RE: Psychrometric Chart help with Air conditioning.

Is dynamic part of the title? Or is that a type of feature your looking for?

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
How do I give somebody a star???

Also are there charts out there that represent the  Psychrometric chart that I could use.  My one fear is that if I read the enthalpy numbers incorrectly (or any number on the  Psychrometric Chart)it can greatly affect my calculations.  I noticed that even small error can greatly affect the final number.  So I would rather do some interperlation on a chart and get my numbers closer to exact than just use my eye and a ruler on the  Psychrometric chart.

RE: Psychrometric Chart help with Air conditioning.

Yes, there is a set of tables in the ASHRAE Fundamentals Handbook, in the Psychrometrics chapter 6 they are called the Thermodynamic Properties of Moist Air Tables. Then with the use of equation number 32 from that chapter and perhaps a little reading, this may make a little more sense. h = .240t + W(1061 + .444t)
Where: h = specific enthalpy BTU/Lbs of the moist air
       t = the dry bulb temperature of the indegrees F
       W = the humdity ratio of the moist air Lbs     
           Lbs water/lbs dry air
Using the table locate the W value and multiply it by the  decimal %Rh to get the humidity ratio of the air at that condtion then enter that value into the equation. Solving for h will give you a very close representation of what the chart is providing.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok.  Well I do nto have that book.  I wil llook over my Trane Air Conditioning Manual to see what that has in it.  Otehrwise I may have to look at purchasing the ASHRE Book.

Do you know the exact name of the ASHRAE Fundamentals Handbook?  What I mean by that is does it have a code number or some type of numberign associated wiht it?

RE: Psychrometric Chart help with Air conditioning.

These books are published by ASHRAE on a four year cycle. the name of the book is:
2005 ASHRAE Handbook Fundamentals. Go to Ebay and look under books ASHRAE, often they are there pretty cheap or amazon books may have one don't expect to pay any more than 30.00 to 60.00 dollars for an older edition 2001 will have the same tables as the 2005. You can go directly to http://www.ashrae.org/ to purchace it new but they ask for quite a bit. You might do a seach on Google for a listing of the tables I've never tried that.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

Jasno999, I just looked in the Trane Manual they do have a set a of tables there PG 400 Table 4-1 The ASHRAE Handbook is still a good idea but this might get you through for now. I would copy down the equation from the post as that came out of ASHRAE not the Trane book

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok here is antoehr good question for you.

How do you incorporate leakage?

Example with made up numbers:

-1 room
-Sensiable load 10,000BTUH
-Laten Load 5,000BTUH
-Total load 15,000BTUH (Includes structural, human, infiltration, solar and electrical)

-100% recirculation air system (no outdoor air to the evaporator)

-Supply fan is 1,000 CFM

-Assume that 200CFM air infiltrates the system.


Now how do you factor in leakage?  My thought would be that since we have 200CFM or 20% infiltration that must mean we have aroudn the same amount of air leaking from the room to the outside.  So we can say that leakage is 20%.

I would think we would calculate the leakage BTUH by this equation:

Leakage = Total Load + [Total Load X Leakage %]

or

15,000BTUH + [15,000BTUH X 20%] = 18,000BTUH

IS THIS CORRECT?


If so do I add the extra 3000BTUH that we found for leakage to the sensiable side of the load?  I need to know that cause it will drastically effect my supply air temperature when I go to calculate that using the sensiable load and sensiable equation..

RE: Psychrometric Chart help with Air conditioning.

(OP)
Or maybe I am not supposed to factor leakage into the equation at all???  If not let me know...

RE: Psychrometric Chart help with Air conditioning.

If you know the sensible and latent loads in BTU/HR as stated and that included infiltration load then the 200 CFM that you mention had to be already factored in the way I see it. The 200 CFM had to be calculated using the amount of crack based on window and door design along with climate data and average wind velocity. It should have been used to find the BTU/Hr loads for the space. That was all part of the space load design.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
YEs I factored in the infiltration of outdoor air into the space.  

However if you have a system that is usign a mixture of outdoor air and return air then you are basically adding air to the system and therfore pressurizing the space to some small degree.  In doing that you will then be forcing some of the air out of the room.

So if you have 75% return air and 25% outdoor air then I would think that you are goign to have 25% of the air flow needign to escape the room via cracks in the doors/windows/wall/etc.

I would think even with pressurization you could still have infiltration alone with the leakage.

For example a strong wind on one side of the room could cause some infiltration air to come in.  However since the space is being pressurized then on the other ide of the room you will have air leakign out to the outside.  I woudl think that rate would be the infiltration rate plus the 25% of the flow that is from outdoor air.

DO you see what I am saying?  

I woudl think that since you are losing some of the coolign load to air escaping out of the room that you would have to somehow factor that into the overall heat loading.

How would you do that?  I don't see how that would be part of the infiltration load cause that load consists of a sensiable load that has no association with flow right?  and the latent portion that is multiplied times the amount of return air being sent back to the evaporator.  

I guess I am confused...

RE: Psychrometric Chart help with Air conditioning.

(OP)
In my application we are not sure what the infiltration rate is goign to be.  SO we have to make some assumptions.  At the very least I am buildign a program that will allow us to change the infiltration amount and see how it affects the overall load on the evaporator.

Yo uare correct that the infiltration rate wil laffect loading because the more you have of it the more hot air gets in and the more work the evaporator has to do.

However you are also goign to have air that leaks out. First off if you bring in 25% outdoor air (for example) and send that along with 75% of the return air thru the evaporator and into the room - then you will return 75% of it but the otehr 25% must escape fro mthe room due to pressuriziation of the space and leakage thru the cracks and spaces.

At the same time if you bring in say 50CFM of infiltration air then you will need to get rid of antoehr 50CFM of room air on top of the 25% that is already escaping.

Now I don't think we have incorporated this leakage of air from the room to the outside itno any of our equations.  I see this air that escapes as a loss in my mind.  If you are losing soem portion of conditionid air then that shoudl be a heat loss right???  

Infiltration as I see it is a heat gain to the system.  Leakage is a cooling loss.  It does seem like using both is sort of a double hit to the system but I would think that you need to include it in the math.

Any help here would be appreciated cause I think that thsi is the last step that I am stuck on.  after I have this figured out I can finish my model and I can really look at all the systems I have in great detail.

Thanks

RE: Psychrometric Chart help with Air conditioning.

If the Hvac system is pressurizing the space then you will not have any heat gains due to infiltration, correct. So that the issue of sensible heat and latent heat gains affecting the space load goes away. But in order to obtain that positve pressure to stop this infiltration to the space, we had to bring in outside air, ventilation air through the unit this is seen as a direct load to the unit. The load is still there for the coil, the only difference is we get to treat it before the occupant sees it. As to not knowing what the leak rate is going to be, yes I suppose thats true, but based on the type of structure, windows ,doors,and quality of construction there is data out there that will allow you to make an educated guess as to the possible amount. Here goes my plug for the ASHRAE Fundamentals Manual again.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
I was able to download the ASHRAE manual via my work computer and software my company has set up.  So I hae that in PDF form and I have been readign thru it.  Still I am left with some confusion.


I understand what you are saying.  The load of the outdoor air is seen by the evaporator coil not the room.  That is the reason why the evaporator coil heat rejection load is higher than the room heat rejection load required.

I guess what I see is that the room has heat gains from different things (electrical/strustural/solar/human/maybe infiltration)-  But I woudl think that thru leakage it would also have a heat loss.  Do you have to add that heat loss into the room load calculation somehow?

And I think as odd as it sounds in my application we may possiably have infiltration loads (infiltration air) coming into the space eeven though the space is goign to be somewhat pressurized.  I can't get itto all the details but the space I am dealign with is not goign to be well sealed and it could see some high wind flow rates on oen side.  SO one end coudl see infiltration air and the other end could see leakage of the infiltration air along with the pressurized air due to the intake of outdoor air to the evaporator.

So do we factor leakage (from inside to outside) into the room load calculation at all????

How exactly?


All the leakage data and calculations I see the the Trane manual and in the ASHRAE manual talk abotu infiltration and the leakage of air into the space.  I need to knwo about leakage out of the space to the outside and hwo to factor that in.

Again maybe my head is screwed up fro mal lthe calculations and I don't need to do that- but I would think that I would.  I would think in the end that the room air that is escaping from the room to the outside should be taken as a coolign loss and should somehow be factor into the room heat loading.

RE: Psychrometric Chart help with Air conditioning.

(OP)
Like on page 34 of the trane manual (bottom of the first column and top of the 2nd column).

It says:

"When outdoor air is taken itno a central air handling unit... pressure in the room can eaisly build up high enough to offset the sind pressures.  This stops infiltration; actually, air is leakign out.."

Ok in my case I think I will still have some infiltration along with a pressurized system that needs to push air out of the room (or leak air out).  However there is not statment after this one to describe how that leaky air is supposed to be taken into account....

RE: Psychrometric Chart help with Air conditioning.

(OP)
On top of those questions here is another one.

IS it possiable to have a higher heat load for the room than for the evaporator in a given problem.

Right now I have a situation where my total room load is like 22,000BTUH where 20,000 is sensiable and 2,000 is latent.  

So I use the sensiabel portion to find the disscharge air temperature.  After finidng that I use the discharge air conditions compared to the mixed air conditions enertign the evaporator to find the total heat lost for the evaporator using the Qt=lb/hr X (delta Enthalpy) and I am gettign a evaparator load of 19,000BTUs which is less that the original load of the room.


IS that possiable?

RE: Psychrometric Chart help with Air conditioning.

#1 First understand that if you are presurizing the room the only way you can do it is to bring in outside air, this outside air is part of the mixed air you are seeing at the coil so the energy to heat or cool this has been accounted for in your earlier calculations. Go to the HVAC forum and check out the FAQ there, question 4.
  #2 If your situation is unique in that even with resonable building pressurizaton you are still going to get some infiltration due to unusual crack conditions and high wind velocities you will have to find a way to calculate that amount. I'am not sure of how to do that but if you post in the HVAC forum somebody with more exposure to this maybe able to help.
  #3 Maybe it was done or maybe you overlooked it but when or how did you find the CFM. After you found your supply air temperature, using the SHR. You would use the supply air temperature and the room air temperature to find the CFM.: BTU/Hr (sensible)÷ (1.08 x ΔT) = CFM Then use the CFM and the supply air enthalpy and mixed air enthalpy to find the total load of the coil.
   #4 If your mixed air condition has an enthalpy value less than the space condition but greater than the supply air enthalpy then you could have a load at the coil less than the load of the room. Isn't that what the economizer cycle is all about? Or it is possible you ommitted finding the CFM using the sensible BTU/Hr [&Delta];T
  

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
Well as we had discussed earlier looking at the design conditions and the real world are two different things.

Basically I had my CFM set for me by the system suppliers.  THe CFM they used is close to what I found for my worst case condition.  But I had the CFM already ste upo and I used it to find out what the supply air temperatures had to be for the system being proposed.

I also did look at the system based on purly design and found my CFMs the way you wanted me to.  However those CFM were al ldifferent for each outdoor air condition.  My fan is only goign to have one speed and since I was given the fan info I used it to solve for SA temp and then used that data to solve for evaporator Q.

But in doing that I am not using the SHR.

What I did was this:

FOund my total load and my breakdown of sensiable to latent load.

then I used: Temp Supply Air = Temp Room air - [BTU/Hr (sensible) ÷ (lb/min X 60 X 0.24)]

Which is Tsa = Troom - [Qs/(flow X Constant)]


So using that I found my supply air temperature and made the assumption that it was coming off the coil at 95%RH.

I then wrote an equation to solve for my mixed air conditions (temperature & Humidity):

Temp mixed air = [Room temp X % recirc]+[Outdoor Temp X (1-% recirc)]

And I then used the humidity (grains/lb) to calculate my humidity values:

Humidity Mixed (Gr/lb) = [Room Humidity(gr/lb) X % recirc]+[Outdoor Humidity(gr/lb) X (1-% recirc)]

Knowing the mixed air humidity in grains per pound would then allow me to use the charts from ASHRAE to determine what the Relative Humidity was for each point. (Basically at the mixed air temp you can find the grains/lb at saturation and use that to devide the mixed air humidity you just found to get a %.

Now I had the %RH and could use that in the equation you gave me earlier to find the enthalpy:

h = .240t + W(1061 + .444t)

When I had the enthalpy of the mixed air I would then use it along with teh enthalpy of the supply air in this equation:

Q(evaporator) = Flow(lb/hr) X Delta h(btu/lb)

And that is how I found my evaporator load.


Give me a quick check and let me know if you see somehtign wrong in that process.

I understand that the outdoor air is taken into accoutn at the evaporator so I do nto need it in the room loading.  Was hard for me to grasp that but I get it now.







RE: Psychrometric Chart help with Air conditioning.

The formulas amd the method all seem to look good to me. Do the numbers work out when plotted on the psychrometric chart, using the vendors CFM?

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
When you ask if the numbers look good when plotted out what exactly do you mean?

I mean I tried to plt them out but I don't knwo if they look good or not.  I had a SHR line that went thru the due point line?  Is that ok?

Tell me what sort of things I need to look for if I plot the points and tell me how you would plot your points out.

RE: Psychrometric Chart help with Air conditioning.

I don't mean to be so vague, I just don't have enough data to know what results your getting. We've thrown a lot of examples around for heat loads and CFM, but I'm not sure what was a senairio and what was a valid number?
   Why not try using the formulas and psychrometric charts on a system that is all ready up and running? It will give you some good practice and allow you to verify your calculations against a functioning system.

 

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)
That is good advice.  However I do nto have a system up and runnign tht I could use as a model.

I know that I have been vague but I can't tell you all the details of what I am working on.  That is why I was throwing out all the different examples.

I have a feeling that something is off or wrong because my results are not matching up with what I have received from some known AC providers.

At the same tiem maybe my numbers are correct and I need to ask some questions.  I am pretty confident I used al lof the proper equations in the proper places.  But when I get my final Evaporator loads I am seeing some different values than what I am being told.

So I am goig nto review my calculations again today to see what I can come up with.  Then I wil ltry to get anotehr set of eyes on it where I work.  After that hopefully we can coem to a conclusion one way or the other.

My concearn is that there is a lower temperature condition which high humidity that is turning out to be the most critical point requiring the most heat rejection at the evaporator.  It makes sense cause the latent load at the evaporator for this condition is high.  However the evaporator load for this condition turns out to be about 10,000BTU higher than for the other conditions that I have.  And that is higher than what any of the systems being offered can do.  So it comes down to one condition that has me scratching my head and then looking at that along with the offered systems to figure out what is goign on.


I will post more questions if something hits me while I am reviewing my numbers.

RE: Psychrometric Chart help with Air conditioning.

(OP)
Ok I have come to a conclusion.  Somehting is definitly wrong with my math.

I did all the the stuff you have told me to do.  I found my disscharge temperatures using the sensiable load required for the room.  Then I used the supply air temperature and mixed air temperature and humidity to get the evaporator Q.

However I have that one ccondition that is a high temperature and a high humidity.  Say 102F and 100%RH -  

When I do the math I get a disscharge temperature of around 64F.

However I was told the room conditins could be maintained at 80F and 50% RH.  When you go and plot this on the psyh chart you get a line that slopes downward from left to right.  Meaning somehow between the supply air and the room air we are losing humidity????  That is not possiable cause we should be gaining humidity here.

I am confused.

What I am finding is that outdoor air humidity effects the evaporator load the mixed air enthalpy and the latent side of the room Q.

Room Humidity effects the evaporator load the mixed air enthalpy and the latent side of the room Q.

Since these do not affect the sensiable load at all and the sensiable load is the only number being used to find the supply air temperature using Qs=Flow(lb/hr)*0.24*deltaT   That means the supply air temperature and humidity is never affected by the laten loads (inside or outside the room).  Therefor the evaporator is the only item that is affected by leten loading.  IT makes sense until you realise that if you have supply air coming into the room and it is drying out (losing humidity) as it gets heated up to the room temperature of 80F that makes little to no sense and it tells me that somehting is wrong in the equation.

If anyhitng we are addign moisture to the room via human presperation and infiltration air.  I realize that the evaporator is supposed ot be cooling the air and gettign rid of that moisture but I am confused as to how I lose moisture as my supply air gets heated back up.  I would think in order to do that you would need some sort of dehumidification device which we do nto have on this project.

RE: Psychrometric Chart help with Air conditioning.

(OP)
What it comes down to is the fact that I do nto see anywhere that my latent load for the room is used in any of my equations???

I mean I use equations to find my latent load associated with infiltration air and I use equations for fidn the latent load due to human sweat.

However I never use the latent load anywhere in any of the future equations.  Somehting is wrong with that cause that has to be factored itno the evaparator load but it is not.

In the end the only things that affect evaporator loading in my worksheet are the mixed air and supply air conditions.  Mixed air in my situations is jsut a mix of outdoor air and return air.  But return air is being defined by the suppliers as a temperature and humidity they are telling me they can maintain.  

I need to factor my latent loads into this worksheet somebow and the calculations that oy uhave given me don't do that.

What do I do?

RE: Psychrometric Chart help with Air conditioning.

Some of these numbers are way off the map, 102 degrees, 100%Rh. And I'm having a hard time keeping up with your train of thought. Several statements caught my eye, for example;
 #1. "I realize that the evaporator is supposed to be cooling the air and getting rid of that moisture but I am confused as to how I lose moisture as my supply air gets heated back up."
    When you cool hot moist air you remove sensible and latent heat (de-humidify), when sensible heat is added to the supply air due to space loads it's temperature is raised this process line will be a straight horizontal line.  The humidity ratio remains the same, and the %Rh will drop, under normal conditions.
#2. However I have that one condition that is a high temperature and a high humidity.  Say 102F and 100%RH -  

When I do the math I get a discharge temperature of around 64F.

However I was told the room conditions could be maintained at 80F and 50% RH.  When you go and plot this on the psyh chart you get a line that slopes downward from left to right.  Meaning somehow between the supply air and the room air we are losing humidity????  That is not possible cause we should be gaining humidity here.


You are correct, it is not possible. I think it’s possible that you have exceeded the design of the coil Or the high temperature and high humidity have made SHR to steep. To find out what will actually happen, plot your SHR line  from the 64 degree 95 %Rh supply air condition up and to the right and where it intersects with the 80 degree DB temperature line. Ideally it should intersect at 80 degrees 50 %Rh. I don't see that happening with your given conditions. This will be the room condition based on the supply air and space loads.
The Trane Air Conditioning manual has some examples on this type of problem and how to solve them starting in Chapter 5 page 106 through111. Take some time to read that and get familiar with what their saying. Examples 5-31 and 5-32 may answer some of your questions.

I'm not a real engineer, but I play one on T.V.
 A.J. Gest, York Int./JCI

RE: Psychrometric Chart help with Air conditioning.

(OP)

I have been readign the book and tryign to figure this all out but I am just spinning my wheels at this point.  I will look again tomorrow but I have been going nuts abotu this stuff.

I tried to break things down again.  I was lookign at it from jsut a design standpoint.  Jsut tking a CFM and using it to get all my other figures.  And trying to set the equations so that everyhintg was affected by the cfm changes.  I don't know.  I am hoping by doign that I can look at my required evaporator Q for each temp/humidity range on outdoor air.  Then I can see what kinds of Q i have at each condition.

I jsut don't know.  I just can't tell is my sheet is workign right and if I have it configured correctly or not.  

RE: Psychrometric Chart help with Air conditioning.

Jasno,

I dare say you didn't try to understand the basics. Revisiting Yorkman's FAQ will help you once again.

With a discharge temperature of 64F (and nearly saturated), it is thermodynamically impossible, without including any other equipment in the system, to get to a control condition of 80F and 50%, even in the case of purely sensible room loads.

It is a superfluous design to consider the worst possible condition in selecting a refrigeration device. ASHRAE did these kind of studies with respect to the ambient conditions and you will get the details from weather data. If you are in a tropical region, selecting 1% conditions of WB and MCDB gives you a good design. If you are in a dry area, you can base your selection on either 0.4% of DB and MCWB (mean coincidental wet bulb temperature) or 1% of the values depending upon your willingness to spend.

I reiterate, again, that you can acheive the off coil temperature constructing your psychrometric process backwards. In simple terms, by drawing a line from room condition to the saturation curve with a slope of SHR.

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