steam capacity needs
steam capacity needs
(OP)
How much (lbs/hr) 15psi steam is needed to heat water from 65-F to 180-F? The water recirculates through a SST - shell and tube HX (6"x36") at 25gpm. This, 115, is the maximum delta T condition. Single pass heating is not desired for this, perhaps 3 or 5 passes. Thanks.





RE: steam capacity needs
Convert 25 gpm of water to lbs/hr of water. Water has a heat capacity of 1 BTU/lbF. Calculate the heat required to raise it from 65F to 180F.
Assuming your steam is saturated, look up the latent heat at 15 psi from the steam tables and calculate the amount of steam required. The latent heat should be around 1000 BTU/lb if you just want an approximate idea.
If the steam is superheated on the inlet, look up the inlet enthalphy, look up the saturated enthalphy of the condensate. And while I agree it's possible to somewhat subcool the steam condensate, it won't affect the amount of steam you require very much.
RE: steam capacity needs
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: steam capacity needs
RE: steam capacity needs
Fill me in maybe I'm missing something. I figured that it took 1,437,500 BTU/Hr to take the water @ 25 GPM from 65 degrees to 180 degress. The latent heat value of 15 PSIG steam is 945 BTU/Hr. Help me get this staight the last thing I want to do is mislead someone
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: steam capacity needs
Could you expand your calculation or tell us what are your assumptions? Yorkman got it right.
Enthalpy of sat. steam at 29.7 psia is 1163.94 btu/lb
Ethalpy of water at 180F is 148.041 btu/lb
Rate of heat to be transferred to water is 500x25x(180-65) = 1437500 btu/hr.
So, steam flowrate should be 1437500/(1163.94-148.041) = 1415 lbm/hr.
If there is no subcooling of condensate, enthalpy of water at 249.7F is 218.325 btu/lb and steam requirement is 1437500/(1163.94-218.325) = 1520 lbm/hr.
RE: steam capacity needs
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RE: steam capacity needs
That could indeed be a reason, only that chicopee's assessment is 31.6% greater than quark's. There are other possible reasons such as different T,P conditions, wetness of steam, misreading of tabulated enthalpies, etc.
Let's chicopee justify his findings, so we wouldn't be obliged to speculate anymore.
RE: steam capacity needs
25gpm X 8.5 lbs/gal = 210.75 lbs/min (Actually 8.5 s/b
8.43)
210.75 lbs/min X 115 btu/lb = 24,236.25 btu/min
=1,938417.5 btu/hr
145,417.7/.75=1,938,900 btu/hr (.75 is the assumed efficiency of HX since you did not specify the type of HX you plan to use- so 75% is a good number for now)
1,938,900 btu/hr / 970 btu/lbm = 1998 lbm/hr
So Yorkman, you are on the right track but you are assuming a 100% efficient HX which is not the case for any type of HX.
RE: steam capacity needs
Chicopee, apparently, your hour has ~80 minutes!
Whatever the exchanger selected, an enthalpy balance is an enthalpy balance, and if you have reasonable steam trapping, no steam would escape or by-pass the exchanger without heating the water. Agree ?
RE: steam capacity needs
RE: steam capacity needs
Any comments?
RE: steam capacity needs
asv80, I suggest you read and digest TD2K's posting above.
RE: steam capacity needs
I was using 8.33 Lbs/gal. I did not correct for the water temperature being 65 degrees, but after looking up the density at 65 degrees I'd say at 8.32 I was close enough. I used 945 BTU/Hr for the latent heat value of steam at 15 PSIG or 29.7 PSIA, from the original posting. I saw that you used 970 BTU/Hr. which would be at 0 psig or 14.7 psia. I never factored in any efficiency of the heat exchange I was mearly calculating the theoretical steam useage.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: steam capacity needs
I completely understand the posting by TD2K.
But this is what I understand from the initial question:
1) water is recirulated through the exchanger
2) delta T of 115 is not required in single pass
3) 3 to 5 passes suggested
I think the confusion may be what the "pass" stands for - likely 3 pass recirculation means 65 to 103, 103 to 141, 141 to 180 which is why I gave that value of about 500 lb/h.
If this is not the case, then TD2K's value would be used.
Would be glad to correct myself if wrong.
RE: steam capacity needs
Asv80, this is my interpretation.
You are speaking of steam consumed per heating step (or pass). And we are speaking of the total theoretical (*) amount of steam needed which wouldn't be altered by the number of steps.
The total could even increase, as chicopee says, if by recirculating the partially-heated water there are heat losses which would need more steam to compensate.
(*)As quark says, no condensate subcooling, only latent heat, considered.
RE: steam capacity needs
Maybe mjf1036 found his answer and we are discussing it for only academic interest.
I agree to yours and others value as the total steam required. (it may vary based on what you assume for the latent heat, density of water, specific heat, etc - the differences may be minor).
My perspective on the above problem being, if it were only recirulating heater and we supply the calculated value (1500-1600 lb/h) of steam into the exchanger, the 25 GPM water should enter at 65 and come out at 180 (give or take losses).
The total mass of steam would remain same but the mass flow rate varies depending on time to complete. To me it seems why supply more steam and complete the process earlier than required.
My two cents only.
RE: steam capacity needs
Since energy has to be conserved, what is gained by one fluid has to be given up by the other fluid. A 75% 'efficiency' factor used to increase the amount of steam required (or any other fluid) is not correct.
RE: steam capacity needs
Try calculating the duty required to heat 25 gpm of water in a single pass from 65F to 180F versus your way. You'll find it takes exactly the same amount of energy.
RE: steam capacity needs
I certainly agree that the total energy is same. Absolutely no doubt.
However when you calculate flow rates, you need to find out per pass otherwise its significance would be lost. The total mass of steam required is same (which comes from the total energy as you pointed out earlier). However, when time is taken as a factor (3 or 5 passes), that total mass needs to be divided and supplied. This was what the original question required.
Another aspect of this is that it will directly reflect on the surface area requirements. Single pass 65 to 180 will need more area than 3 passes. Even if all the steam required is supplied it may not heat the water to the required final temp if does not have enough area.
Also right that there is no efficiency with regard to heat exchanger.
Every step of you answer and approach is right except that was not asked in the question.
I highly respect your comments (TD2K, 25362, etc) as I have benifited from your views in numerous other posts. However, I beg to differ in this one.
Regards,
ASV
RE: steam capacity needs
ASV, your point has been clarified.
At least in theory, factors such as water hardness and changing thermophysical properties, as well as diminishing LMTD's, may affect the resulting ΔT's on the water side, so the steam consumed per pass may not be the same for a given exchanger and equal water mass flow rate.
RE: steam capacity needs
1. The mass flow rate of steam is as calculated by Yorkman irrespective of type and configuration of heat exchanger (if the steam can be subcooled, the calculated value corresponds even to the direct injection of steam).
2. A safe bet can be to consider a HX as 100% efficient. Atleast that is the design criteria for any heat transfer application. (heat lost by a hot medium = heat gained by cold medium = OHTC times HX area times LMTD). The only parameter that will affect the heat transfer is dissoved gas. It is prudent to trap these off rather than considering some cushion in heat transfer calculation.
3. If the steam traps are properly maintained, there is nothing like more steam flow rate. Saturated steam condenses at such a rate that is exactly required and mass flow rate of steam in is exactly equal to mass flow rate of condensate out.
4. The no. of passes neither increase nor decrease the steam flow rate required. Suppose, if we have to heat 25gpm of water from 65 to 180F, for every minute, 25 gallons of water is heated by using 23.33lbm/min (or 1415 lbm/hr)of steam.
If we have three passes then each pass should take place (at 25gpm) for 1/3rd of a minute and not for one minute.
5. No. of passes doesn't alter the heat transfer area. The no. of passes only makes the size of HX sensible in terms of length. A particular heat transfer application requires a heat transfer area of 6 sq.mtrs of which 3 mtrs is the length, 3 passes can reduce the HX length to 1 mtr.
RE: steam capacity needs
If I understood correctly ASV's posting, the original intention is to use a small HX (say, 10 ft2) for heating a certain amount of water step-wise (ie, passes) by recirculating it through the HX several times at the rate of 25 gpm.
This entails collecting the partly-heated water and pumping it through the same HE after each heating interval. Such an arrangement may be more or less intricate and result in additional heat losses calling for an increased total steam consumption.
It would, however, allow to carry out the job with a small HX, a protracted operation, and a reduced hourly steam rate.
RE: steam capacity needs
Darn it any how, you beat me to the point again. Heat transfer surface is heat transfer surface. With Asvs' theory the more passes I put in the less steam it will take to make delta T, heck put in 10 passes I could cut the steam to 150LBS/HR! I agree with Quark, A single pass Hx Y feet long produces a delta T of 115 degrees, The only gain that I achieve by increasing the number of passes to produce the same delta T is the reduction in length. The heat transfer areas are going to basically stay the same, Im just putting it in a shorter package.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: steam capacity needs
Yorkman, it seems to me it is all a matter of semantics: you speak of HX (mechanical) number of passes, and asv80 is speaking of passing (ie, recirculating) the water in given lots several times, at the same flowrate, through the same small HX, until it reaches the required temperature.
Did I rightly grasp the difference ?
RE: steam capacity needs
When you say with 10 passes the steam requirement is 150 lb/h, it would very much so, which implies the surface area requirements is even lesser - a more compact unit at a lesser price. The drawback being, they have to wait more time to get the water at 180F. It would be this give-and-take between how much you desire to pay for a unit vs how long you can wait. In this case, they already have a hx, its surface area is fixed.
As 25362 mentions, "pass" does not mean how many times water circulates inside the hx before actually leaving the hx. Rather it stands for the number of times the water went from a tank to the heater and back.
If you re-read the original post this would make more sense. This being a simple problem is what may make us overlook the actual question and over-analyze.
ASV
RE: steam capacity needs
Steam required is about 1520 #/hr.
RE: steam capacity needs
I will agree that if you count the number or times the fluid leaves the Hx and returns as total passes, yes then you could divide the total BTU by the number of passes and come up with your numbers. In the end it will still require 1,437,500 BTU's.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: steam capacity needs