Condenser sizing

[Sgrios]

I've have been searching for some 'rules' regarding condenser sizing on the forums and in a few Modern Refrigeration books that I own, only to come up with nothing.

I wanted to know if some of the 'experts' could lend some calculations or insight in properly sizing a condenser. I hate throwing parts together and I would rather engineer something to get away from Ebay designed units (If you will); not that there is anything wrong with building a unit like this, it is just not for me.

I cannot remember the terminology that Modern Refrigeration used..

Factors:
Work load (Total amount of watts to be cooled)
Heat being produced by the Hermetic compressor IE. Windings, heat produced from cooling the work load.
Not sure how I would go about calculating the heat being produced by the compressor..

Any help would be great. :yepp:

Jeff

[tiborrr]

Rejected heat of compression + heat load (from the cooled source) = needed condenser cooling capacity (kW)

[Sgrios]

Thanks Tiborrr, but how do I go about calculating rejected heat of compression for a given compressor? The .PDF does not list a value under that name or a similar heading..

[tiborrr]

Isentropic efficency * Power draw would be your best assumption of a rejected heat of compression. Too bad Clemmaster isn't around, I'm sure he'd had an accurate explanation for it!

[Sgrios]

Heat of compression =
Enthalpy of vapor leaving (BTU/lb.) compressor (BTU/lb.) -
Enthalpy of vapor entering (BTU/lb.) compressor

Not to say I understand how to calculate Heat of compression even with the formula in front of me.. :confused:

Layman's terms is what I'm in need of, physics was so long ago..

[SoddemFX]

Condensor and fan assemblies are rated for a specified capacity with a condensing temperature delta (CTD) above ambient, the normal CTD is 15K, the conditions used don’t usually include any sub-cooling i.e. the condensor outlet will contain saturated refrigerant.

In this example a condenser is rated for 2kW of heat rejection at 15k CTD. The refrigerant I am using is R507, the ambient temperature is +25C and my rejected heat load is 2kW:

The saturation temperature is 25C + 15C = 40C, therefore the saturation pressure will be 258PSIg at 2kW of heat load

If the heat load is reduced, the CTD will reduce proportionally. So with 1kW of rejected heat the saturation temperature will be 25C + ((1 / 2) * 15)C = +32.5C and the saturation pressure will be 212PSIg

The heat rejected by the condenser in a real system is effected by the heat load on the evaporator, the heat ingress into the suction line (which can usually be ignored for a single stage) and the power consumed by the compressor but a very important part is the heat rejected by the compressor shell.

A normal CPU single stage compressor such as a NL11F will experience something around 30-40% of the total heat rejection from the compressor shell and this is obviously a very significant factor.

Because of this and the lack of information available on compressor shell heat rejection, i think it is best to calculate the expected condenser heat load using a bit of “finger waving”.

For example, I may not know good figures for compressor shell heat rejection but I do know that I built a similar system and the discharge temperature was about +70C . Using this temperature with some simple flow rate calculations I can predict with fairly good accuracy what my condensor heat load will be.

For example if your target system performance is -40C at 250W, the mass flow in the system will be 2.3 grams per second (250 / 108000). Using the +70C figure for the condenser inlet the total heat rejected by the condenser is 405W (175000 * 0.0023)

You choose your condensor based on the above bearing in mind the CTD you want or need for your system and compressor.

It is an iterative process (because as you close in on the real figures, the values change slightly) which involves a bit of guessing but it does work, you can also see why there are diminishing returns for larger condensors.

Tom

[captaincascade]

I think you might be the first person in this forums history actually doing the math to calculate propper condenser sizing and actually puchasing it. Most people use what they got or go for the smallest they can get. way ta raise the bar!!! :)

[Sgrios]

Condensor and fan assemblies are rated for a specified capacity with a condensing temperature delta (CTD) above ambient, the normal CTD is 15K, the conditions used don’t usually include any sub-cooling i.e. the condensor outlet will contain saturated refrigerant.

In this example a condenser is rated for 2kW of heat rejection at 15k CTD. The refrigerant I am using is R507, the ambient temperature is +25C and my rejected heat load is 2kW:

The saturation temperature is 25C + 15C = 40C, therefore the saturation pressure will be 258PSIg at 2kW of heat load

If the heat load is reduced, the CTD will reduce proportionally. So with 1kW of rejected heat the saturation temperature will be 25C + ((1 / 2) * 15)C = +32.5C and the saturation pressure will be 212PSIg

The heat rejected by the condenser in a real system is effected by the heat load on the evaporator, the heat ingress into the suction line (which can usually be ignored for a single stage) and the power consumed by the compressor but a very important part is the heat rejected by the compressor shell.

A normal CPU single stage compressor such as a NL11F will experience something around 30-40% of the total heat rejection from the compressor shell and this is obviously a very significant factor.

Because of this and the lack of information available on compressor shell heat rejection, i think it is best to calculate the expected condenser heat load using a bit of “finger waving”.

For example, I may not know good figures for compressor shell heat rejection but I do know that I built a similar system and the discharge temperature was about +70C . Using this temperature with some simple flow rate calculations I can predict with fairly good accuracy what my condensor heat load will be.

For example if your target system performance is -40C at 250W, the mass flow in the system will be 2.3 grams per second (250 / 108000). Using the +70C figure for the condenser inlet the total heat rejected by the condenser is 405W (175000 * 0.0023)

You choose your condensor based on the above bearing in mind the CTD you want or need for your system and compressor.

It is an iterative process (because as you close in on the real figures, the values change slightly) which involves a bit of guessing but it does work, you can also see why there are diminishing returns for larger condensors.

Tom


I think you might be the first person in this forums history actually doing the math to calculate propper condenser sizing and actually puchasing it. Most people use what they got or go for the smallest they can get. way ta raise the bar!!! :)

Thank you very much Tom.
Thanks Captaincascade.