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.

 

[25362]

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 [Δ]T'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.

 

[quark]

I think a summary of all the posts (and including my comments) may clear up some redundant confusion.

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.

 

[25362]

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.

 

[Yorkman]

Quark,
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

 

[25362]

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 ?

 

[asv80]

Yorkman,

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

 

[toothless]

Don't consider the sensible heat in the condensate thus the exit enthalpy is 218.4. The exchanger is virtually 100% efficient.

Steam required is about 1520 #/hr.

 

[Yorkman]

I'm think we"ll have to agree to disagree, I've always considered that the number of passes when related to a Hx meant the number of times the liquid traversed the length of the heat exchanger. A 1 pass Hx went in one end and out the other, 2 passes in one end to a return bend and back out the same end this end has a divider plate, 3 passes would enter and leave at opposite ends and have two returns bends. On flooded evaporators and condensers the return bends are replace with divider plates at the water boxes.
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

 

[chicopee]

OOPS--So it seems 25362 which brings out an interesting anecdote. I was to estimate the amount of condensate waste from a direct fired steam generator used in a pre-cast conrete plant. Much to my chagrine the time piece that I borrowed from the plant foreman happened to be calibrated for 90 sec instead of 60 sec for one full revolution of the watch needle.