Choosing the operating pressure for a heat exchanger system is an important design decision. The choice of pressure will impact the selection of the heat exchanger and the steam trap for your application. In many cases, adequate performance can be obtained over a wide range of system pressures. However, failure to consider all of the system characteristics and components can lead to inefficient operation.
Selecting the Heat Exchanger
For purpose of this discussion, assume that an engineer is designing a heat transfer system to deliver 70 GPM of water heated from 60°F to 120°F. The steam available to the temperature regulator is 100 psig. The system will use a steam regulator with both a pressure and a temperature pilot. Unsure of the optimal pressure, the engineer sizes the components for two different potential designs for comparison — one using 15 psig steam and the other using 50 psig steam.
The engineer goes to ESP-PLUS, a handy selection tool, and for the given conditions, makes the following heat exchanger selection:
|Steam to Water
|1997 List Price|
As expected, the heat exchanger with the higher pressure can transfer the required energy with less surface area. Therefore it is slightly smaller and less expensive.
Selecting the Steam Trap
The engineer plans to use a float & thermostatic steam trap to obtain quick venting and good response to fluctuating loads. The steam trap should be selected based on a 1/2 PSI pressure differential any time a modulating control valve is used. The condensate load can be estimated using the formula:
(GPM) X (0.5) X (Temperature Rise °F)
The ESP-PLUS Steam Specialties program applies a 1.5 safety factor (SF), and also takes into account the fact that the latent heat of vaporization — the amount of energy removed from each pound of steam — is slightly less at higher pressures. The ESP-PLUS Steam Specialties selection gives the following trap suggestion:
|F&T Steam Trap||1997 List Price|
The considerably larger orifice on the lower pressure traps allows a much smaller trap to be used with the 15 psig design. Just as important, the price difference is very significant.
Don’t Forget Flash Loss
Both of the proposed systems will suffer from flash loss. Condensate leaves float & thermostatic traps as saturated liquid, and at the saturation temperature for the pressure ahead of the trap. The condensate has too much energy for the low pressure on the downstream side of the trap, and the result is flash steam, removing energy from the condensate and cooling it to the saturation temperature corresponding to the low return line pressure. With the steam trap draining to a vented condensate receiver, the flash loss is 9.0% for the 50 psig heat exchanger and 3.9% for the heat exchanger operating at 15 psig.
Based on a steam cost of $7.50 per 1,000 lbs. of steam, the first year expenses (assuming the system operates 8 hours per day and 240 days per year) are:
|Cost of Trap
& Heat Exchanger
|Cost of Flash Loss||Total Cost|
The difference in results is very impressive. In addition, a low pressure system will be more likely to provide better control and response to load changes. For these reasons, Hoffman Specialty recommends using low steam pressure whenever possible.
Reprinted from TechTalk January 1998