steam capacity needs 4

[mjf1036]

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.

 

[TD2K]

The number of passes has no impact on the amount of steam required though it can affect the exchanger design of course.

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.

 

[Yorkman]

Assuming the 15 psi steam is 15 psig and not 15 psia, you will need about 1500-1600 Lbs/hr (1521 Lbs to be exact.)

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

 

[chicopee]

Yorkman, who ever wrote your script is trying to undermine your performance. The answer should be closer to 2000 lbm/hr(1998 lbm/hr to be exact with ass-um-ptions)

 

[Yorkman]

Chicopee,
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

 

[quark]

chicopee,

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.

 

[katmar]

chicopee, this is a regular problem. You are obviously working in Imperial gallons and Yorkman is working in US (or Queen Anne) gallons. An Imperial gallon is about 20% bigger than a US gallon. Time to switch to SI Units?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

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.

 

[chicopee]

OK, here is my answer:

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.

 

[25362]

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 ?

 

[asv80]

I think you may require much lesser steam flow like 500 lb/h as there are 3 or more passes. Any Comments?

 

[asv80]

It may only require about 500 lb/h steam as it is not a single pass heating.

Any comments?

 

[25362]

asv80, I suggest you read and digest TD2K's posting above.

 

[Yorkman]

Chicopee,
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

 

[asv80]

25362,

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.

 

[25362]

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.

 

[asv80]

25362,

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.

 

[TD2K]

HXs are effectively 100% efficient. There is some heat loss to the ambient surroundings but I'm guessing it's a small fraction of the heat exchange that occurs within the two fluids.

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.

 

[TD2K]

"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"

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.

 

[asv80]

TD2K,

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