How do I determine the efficiency of heat exchanger?

[murphymok]

Hi,

I am trying to figure out how much improvement in the HX efficiency after chemical-cleaned the water tube. It's steam-water HX. Any thoughts?

Thanks in advance

 

[TBP]

If you know how much steam it took to heat the water (GPM and delta-T) before the cleaning, then you could compare how much steam is needed to do the job after the cleaning. You may also need the water side pressure drop, before and after the cleaning. You'll need less pump power to move the water through a clean HX. Steam will likely be the big one, though.

 

[imok2]

This may sound simplistic but if you know what the delta "T" was at full flow before cleaning and delta "T" after cleaning, this should give you an indication of improved eff.

 

[davefitz]

using "compact heat exchanger theory", the effectiveness of a heat exchanger is based on the fluid stream with the lowest value of W*Cp, where w= flow and Cp= heat capacity. Alternatively the flow stream with the highest temperature change is the governing fluid.

I asssume the steam is condensing at a saturation temperature Ts, based on the pressure of the steam side of the Hx, Ps. This means the governing fluid is the air.

Effectiveness =e= (Tao-Tai)/ (Ts-Tai)

Since the ratio of heat capcities is almost infinite for the case of a condensing fluid ,the effectivness can also be directly calculated by :
e= 1-exp(NTU), where NTU= U*A/(Wa*Cp,a)
where U= overall effective heat transfer coefficient, including effects of fouling and flow maldistribution.

If you measure the e using test data fromthe first equation, you can work bakwrds to the effet overall heat transfer coeficient U, and see how this had improved by the cleaning operation.

 

[25362]

It is indeed difficult to compare H/E performances when flowrates vary and are unknown. If, as I assume, water flow rates are in the range of 1.5 L/s, I was thinking of the following possible experiment.

At both opportunities, when taking the unit out for cleaning, and after the chemical cleaning is finished, but before it is returned to duty, run the exchanger with water in such a manner that constant -known- flow rates can be obtained. There are simple methods to measure water flow rates which do not require the installation of instrumentation. If it isn't worth or otherwise desirable to throw away water to drain, collect it for reuse.

Measure temperatures. Compare duties. Estimate efficiencies.

BTW, such an experiment can be done with various water flowrates, to create an ample set of data for comparison.

Please comment.

 

[tickle]

What type of HE is it? - Assuming Shell and tube

The heat transfer is Q=UA.DTlm

and U=(1/ho+1/hio+wall resistance+fouling factors)^-1

For steam on the shell side the ho is 6000 to 800 W/m2K, including fouling factor. Wall resistance and hio will be the same for clean and dirty.

Cleaning will remove the tubeside fouling factor. Lists of fouling factors are given in various chem eng books.

The method of control needs to be considered because if it is a control valve on the condensate side then the HE will be flooded hence reducing the effective heat transfer surface area.

 

[tkdhwjd]

Design U from the exchanger manufacturer has a built-in fouling factors for shell and tube sides. If feasible, U could be monitored to see how fast/much deviates from the design U and attribute the deviation to foulings. U before and after the cleaning can be compared to evaluate the effectiveness of the cleaning... at least that how I did it.