heat exchanger performance

[honche725]

how do you rate a heat exchanger's performance? what parameters should one know aside from the inlet and outlet temperatures and pressures?

thanks.

 

[tkdhwjd]

Please be more specific what you are asking. Are you asking how to size or monitor performance, or else?

 

[honche725]

hi tkdhwjd! i am referring to monitoring its performance, is it performing according to specifications, etc.

thanks!

 

[rmw]

You kind of need to know what is flowing through both sides of the exchanger, what is the flow rate and the technical parameters of what those fluids are.

Then you can calculate the duty and see if it is performing according to the duty that it was designed for.

rmw

 

[itdepends]

Inlet and outlet temperatures, mass flow and heat capacity of fluids. You don't have to have all of these- e.g. if you've got the heat capacities of both fluids, inlet and outlet temps of both and only one flow- you don't need 2nd one but it helps to cross check.

 

[tkdhwjd]

honche725,

You should be able to get started with suggestions that rmw and itdepends provided. If you are still struggling, suggest listing what data are available.

 

[honche725]

hi everyone! thanks so much for the replies. actually, the heat exchanger im referring to is a cooler condenser. i am cooling compressed air to condense water out of it. i will be able to know the temperatures in and out of both the compressed air and cooling water but i don't have actual info on their flow rates (except those in the spec sheets).

i would appreciate it much if you guys can help me sort this out. thanks.

 

[MJCronin]

honche....

Perhaps this may help....

Almost all heat exchangers are designed to a performance point that is optimum for the thermal cycle that they are in.

Generally speaking, process heat exchangers are designed for optimum generation of product at minimum capital expense. Power plant heat exchangers are selected to generate maximum elctric power based on the thermal cycle which will generate the maximum return on investment for the owner.

That being said, it is important to underatnd that most heat exchangers do not operate exactly on the "optimum design point" because systems are rearely operating exactly on that point.

This makes the testing and rating of the individual heat exchangers difficult, because there always must be an adjustment between the thermal performance observed and that which formed the basis of design....

My opinion only....

MJC

 

[dcasto]

Your only measured variable is outlet temperature. You could look at your air compressor(s) and take flowrate data off of them and compare outlet temperature.

 

[tkdhwjd]

As dcasto mentioned, estimating flow rate of compressed air using compressor data in one method (using compressor curve). Then with temperature data and thermal properties of air/water, you should be able to calculate the duty and transfer coefficient and compare them to the design spec.

Or, you can roughly measure CW rate (using portable ultrasonic meter) and follow the similar steps to measure the exchanger performance.

 

[twr]

I've had similar fun with heat exchangers - there aren't many flowmeters used for cooling water, at least at our plant. I found the flows with centrifugal pump curves and valve Cv calculations using inlet and outlet pressures and control valve data.
We wound up running Ridlime through the exchanger to restore performance to the unit. It had been pretty badly fouled by the cooling water side.
Here's a link to the Cv calcualtions:

http://www.engineeringtoolbox.com/flow-coefficients-d_277.html

 

[toothless]

A common parameter used to measure the performance of a heat exchanger is "approach temperature". This is a measure of how close the cooled fluid (outlet temp) gets to the cooling fluid (inlet temp). For instance, if I get 100 F air out of my exhanger which is cooled with 80 F cooling water, my approach is 20 F. If you'll look at this parameter when the exchanger is clean and new you can get a baseline performance figure.

 

[TBP]

You really can't beat the value of having pressure gauges and thermometers on the inlets & outlets of both sides of a HX. For what amounts to peanuts in added cost on the initial capital project, a few pieces of very basic instrumentation can yield pretty much all of the info needed to calculate the actual conditions such as flows. The heat exchanger manufacturer will be easily able to run some calcs, based on this basic info, and the model of HX that's in service.