Help in designing of cooling coil of finned tubed heat exchanger!!!!!

[sageer.ahmed25]

2TR split ac evaporator design........Refrigerant: water
I want to know the design procedure as in how to calculate the water side heat transfer coefficient....what correlations to use??....and eventually how do i get overall heat transfer coefficient...

[botrous]

Dear Sageer.
Designing a heat exchanger is not as simple a you would think.

But here are some hints for you .

A 2 tons evaporator usually have an 800 cfm airflow, the speed of the air passing on the heat exchanger should not exceed the 400 fpm (the speed is limited by the speed of air that will not carry water) , those the face area should be aroung 2 sqf.

Generally a 3 rows coil with 13 fpi will do the job.

Another concern is the pressure drop of the water in the coil, selecting the number of feeds.

Please see the attached selection and note that the required sensible heat load and latent heat load will change the heat exchanger dimensions.

Best regards

[karim]

thanks buddy !

[botrous]

you are welcome . . .

[sageer.ahmed25]

yeah....but in evaporator temperature is kept constant to 7deg C and water inside tubes just changes phase..... and room temp.... is taken to be around 40 deg C which eventually drops to 20 deg C.....based on which correlations to use in finding water side heat transfer coefficient....as it is a forced convection type and eventually get the dimensions as i already have calculated air side heat transfer coefficient......or else is there any other way to get the dimensions of the evaporator for a split ac.....pls help

[lana]

Hi there,

If you are talking about a water coil then there would be no phase change for WATER.

I developed a software to calculate cooling coils many years ago and use it all the time because manufacturing coils is my job but as it's been so long I don't remember the exact formula. Try some of the heat transfer handbooks.
If you have phase change inside the tubes, like R22,... then the situation is much more complicated.

Cheers

[botrous]

2TR split ac evaporator design........Refrigerant: water I want to know the design procedure as in how to calculate the water side heat transfer coefficient....what correlations to use??....and eventually how do i get overall heat transfer coefficient...

if the refrigerant is water, then no phase changes will occure, all the heat extracted from the room to the water will be transfered to sensible heat in the water side.

As for refrigerants (R22 . . .) the heat extracted from the room to the refrigerant will be divided into sensible heat and latent heat.

As for the 2T @ 40C inlet air or 2T @ 20C inlet air , the coil will have different physical dimensions.

Generaly coil are rated using ARI standard.

A 2T coil rated @ ARI standards will give more than 2 T if the air inlet tempreture is higher and less than 2T if the air inlet tempreture is lower.

Carefull design should be considered.

[botrous]

Sageer,
Please see the attached file, I think it will give you a closer look to what you want.

[sageer.ahmed25]

it is a vapor absorption system...and pressure is maintained to keep the water at saturated temperature of 7 degC.....so y cant water be maintained at constant temp......????

[botrous]

Is it an absorption system using lithium bromide and h2o solution ?

What is the evaporator pressure ? is it 0.01 bar ?

regards

[botrous]

deltaH for water to be come vapor in this case 2466 Kj/kg apporoximatly.

Q=mx deltaH

Hope this will help

[sageer.ahmed25]

yeah.....it is 0.01 bar...i did calculate the mass flow rate of water...but i wanna calculate the water side heat transfer coefficient....how wud i approach to that?

[botrous]

See this example Sageer




Evaporator Coil

Model
Typical (1/2 inch)
305 x 920 3r 10f 4c

Air
Barometer 101.325 kPa
Altitude 0 m
Airflow 800 cfm
Velocity 265 fpm
On Coil 27.0/19.5 °C
Off Coil 15.5/14.2 °C
Cond Rate 0.7 g/s
Pressure drop 0.0249 kPa

Refrigerant (Evaporating R718)
Temperature 7.00 °C
Abs. Press 2.39 kPa
Density 979 kg/m³
Enthalpy Change 1988 kJ/kg
Mass Flow 12.6 kg/hr
dP Rate 0.0135 kPa/m
Press Drop 0.113 kPa


Performance
Sensible Duty 17704 Btu/hr
Total Duty 23770 Btu/hr
SHR 0.745

[sageer.ahmed25]

tnx...but i dont get wht i want to know 4m ur reply.....it is the heat transfer coefficient ...which i have to find....so eventually i get area and size of cooling coil (evaporator) based on that....

[botrous]

Of course sageer , you need to calculate the heat transfer coefficient , based on that you define the air flow , you calculate the type pf heat exchanger efficiency , you calculate the needed area and you get the heat exchanger. . .

(it is not as simple as i stated but it will work)

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