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air leaving temperature-psychrometric

air leaving temperature-psychrometric

(OP)
The method of find out the actual leaving air temperature of cooling apparatus, here an example takes an FCU. The chilled water entering temperature 47F, bypass factor .20 and entering air temperature is 75F. Please see the calculation as per the psychometric formula
Tldb=Tadp+bf(Tedb-Tadp), then 47+.02(75-47)=52.6F( 11.4C) is it practical in field?

RE: air leaving temperature-psychrometric

Compare your results with those equations used in the examples shown in the Perry Chemical Eng. Handbook.

RE: air leaving temperature-psychrometric

(OP)
chicopee: can you specific the page number

RE: air leaving temperature-psychrometric

No because Tadp, apparatus dew point temperature is not equal to the entering chilled water temperature. Tadp is higher than the leaving chilled water temperature. You should instead use the manufacturer's rated sensible and latent heat capacity at their rated conditions of entering chilled water temperature, chilled water flow, airflow and entering air temperature and humidity.

Chicopee X = 49 by trial & error in excel.

RE: air leaving temperature-psychrometric

(OP)
lilliput: Tldb=Tadp+bf(Tedb-Tadp), then 47+.02(75-47)=52.6F( 11.4C). Did you mean like that the
Apparatus dew point temperature is not equal to the entering chilled water temperature to the fan coil unit.

RE: air leaving temperature-psychrometric

(OP)
How to find the actual apparatus dew point of chilled water fan coil unit

RE: air leaving temperature-psychrometric

See Carrier Chapter 8 Applied Psychrometrics pages 1-120 to 1-123.

Apparatus dewpoint temperature and bypass factors are now not used in rating fan coils. Google Trane chilled water fan coils and see ratings base on unit size, CFM, EWT, WTR, Entering Air EDM/EWB. The ratings are given in TC (total capacity MBH), SC (sensible capacity MH), GPM, WTR (Water temperature rise, PD water pressure drop ft of water.

SC x 1000 = 1.1 x CFM x (Tedb - Tldb)

Tldb = Tedb - (SC x 1000)/ (CFM x 1.1)


RE: air leaving temperature-psychrometric

(OP)
lilliput1: then i have calculated as below and imaged

RE: air leaving temperature-psychrometric

Correct

RE: air leaving temperature-psychrometric

(OP)
then what is the role of chilled water temperature. in this formula chiller water temp is not subjected.

RE: air leaving temperature-psychrometric

A set of table is specific to an EWT. Notell leftmost column designating EWT

RE: air leaving temperature-psychrometric

(OP)
Can be used the same formula for dx system?

RE: air leaving temperature-psychrometric

DX systems do not have an Entering Water Temperature

RE: air leaving temperature-psychrometric

(OP)
willard3: here we discussed entering air temperature.

RE: air leaving temperature-psychrometric

If you use the rated sensible heat capacity of the DX system at the conditions at which the rating applies yes the formula applies because it is the definition of sensible heat capacity.

RE: air leaving temperature-psychrometric

In my Perry's Chemical Engineering Hdbk,4th edition, chapter 15 under the subject of psychrometry, you should check out the examples to determine suitability to your OP.

RE: air leaving temperature-psychrometric

(OP)
I have imaged the air entering mixed temperature-psychrometric formula


RE: air leaving temperature-psychrometric

So use the table at the entering air temperature of 79.4F instead of 75F to get the correct LAT. You should study and understand psychrometrics. What condition of air (CFM, dry bulb temperature, wet bulb temperature, moisture in grains/lb, points on psychrometric chart) would be needed to off set the space and outdoor air sensible heat and latent heat load. Understand how to plot the process on a psychrometric chart. See Carrier Handbook of Air Conditioning System Design Chapter 8 Applied Psychrometrics.

RE: air leaving temperature-psychrometric

(OP)
lilliput1: I prepared above example based on that same book.

RE: air leaving temperature-psychrometric

The one component missing in your drawing and which was inferred by Lilliput1 is the moisture content. If you can defined some relative humidity, you can plot the entire process on a psychrometric diagram and get the answers needed. Right now the formula that you presented is for dry air which is unrealistic in the real world unless you live in some of the driest places on earth. If you check out some of the used book dealers such as Abebooks, you may want to get a copy of "Mechanical Engineering License Review" authored by R.K. Pefley and D.G. Newman. This soft cover book has sample problems that would benefit your learning.

RE: air leaving temperature-psychrometric

(OP)
chicopee: thank you for your reference, I will try to collect that book. yet, how moisture content influences the dry bulb temperature? the moisture content influences the total capacity of cooling apparatus.

RE: air leaving temperature-psychrometric

The higher air temperature from a regular thermometer(dry bulb temperature), the higher the moisture content. Cold air by contrast can not hold much moisture. Only by studying psychrometric charts, you will understand the relationships between dry bulb temp, wet bulb temp, R.H , moisture content and the enthalpy of air.

RE: air leaving temperature-psychrometric

This is great but it’s academia. We can get a 55°F leaving dew point by limiting DAT to 55°F; or a 51°F leaving dew point via a DAT of 51°F. If you want to get technical, the 55°F LAT with a larger cooling coil bypass factor will have a lower dew point. So you can have a lower coil ADP temperature with less fins per inch and have a 55°F LAT with a 51°F dew point, or a higher coil ADP temperature with more fins per inch and have a 55°F LAT with a 54°F dew point.

To me, this is minutia. As a general rule, if you need a space (e.g., pharmacy or OR) that wants a 68°F indoor temperature and less than 55% RH (dew point of 51°F), supply 52°F air during the summer and reheat the air to the desired room condition. The supply air temperature is the key; the fins per inch, bypass factor and other parameters matter, but are generally academic.

Wait, I might've answered the wrong question again... Sorry, I like going off on tangents.

RE: air leaving temperature-psychrometric

Thank goodness computerized coil selection makes the task so much easier without figuring out apparatus dewpoint and bypass ratio. You calculate the required cooling coil leaving air dry bulb temperature and moisture content to satisfy the following equations:

CFM dehumidified air = (Total Sensible Heat BTU/hr) / (1.08 x (TF ent clg coil - TF lvg clg coil)

CFM dehumidified air = (Total Latent Heat Btu/hr) / (0.68 x (W grains/lb ent clg coil - W grains/lb lvg clg coil)

Total Sensible Heat = Room Sens + OA Sens + Fan Heat Gain + Duct Heat Gain

Total Latent Heat = Room Latent + OA Latent

For space HVAC CFM dehumidified air is usually = Room Sensible Heat Btu/hr / (1.08 x 20) but this may be changed depending on the coil selection.

Select coil type, and fin arrangement and at maximum 500 fPM face velocity to avoid moisture carryover. Try out various available circuit arrangement to pick coil meeting desired leaving conditions with minimum air pressure drop and with less than 20 ft water pressure drop.

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