Calculating the Cooling Capacity Required for a Compressor

[ConfusedEng]

I have problem that I need some help with, I have attempted to figure it out after reading up on compressors and heat-exchangers and tried a few ways to solve it but i am just getting myself confused. If anyone could advise what methods i should use to solving it i would be very grateful.

The problem - (Part 1)
I have a refrigeration system that is used to cool water via a solid aluminium block heat-exchanger (the cold side of the compressor circuit chills the block and the water pipe passes through the block to cool the water inside) the hot discharge line of the circuit sits in a tank of water which is dumped to drain & recharged at a constant flow rate when it gets too hot to remove heat (this tank maintains its temp around 50°C). The compressor, the aluminium block and the water tank are contained in within another enclosure , the aluminium block is surrounded by insulation so is the water heat exchanger tank but the compressor is not and so any heat from the compressor heats up the surrounding air the box, in addition there is also another insulated heat source sitting in the box unrelated to the compressor circuit.

I know the following parameters
The incoming water temperature and desired outlet water temperature, (the required change in temp)
the quantity of water required and flow rate of the water passing through the aluminium block heat-exchanger
the pipe sizes & material, the mass of aluminium, the insulation material properties and thickness around the block
The temperature of the water heat-exchanger, inlet and outlet temps and the quantity coming out to drain. the tank wall material thickness & insulation info
The material of the enclosure it is all sitting in and the ambient temperature outside of the box.
The temperature of the compressor when in operation and the temperature of additional heat source.
and all the materials used and corresponding properties.

So far I have calculated the amount of energy required to cool the quantity of water in Joules and the time available to cool in giving me the Watts of cooling required... I think I should also include the other heat sources as they are also acting on the aluminium block as well considering they are all contained within the enclosure them once i know the energy these other heat sources are adding to the Al Block i must then compare the total watts against the Cooling capacity stated in the compressor specification. or is there more to it than this?
If this is it, how do i assign a value to the extra heat being added to the Al block by the other heat sources?

TLDR:
I am trying to figure out if the compressor I am using is capable of cooling a quantity of water to a specific temperature when there are additional heat sources in the enclosure. What equations should i be using to include all heat sources?

 

[lukaiENG]

Do you have a compressor selection? The compressor selection should be sized to move the quantity of heat from the block (which you have worked out) into the water tank with the tank at the worst case temperature. The compressor will need to be sized to perform this work, and it's input power will correlate to the heat load it provides to the enclosure.
How is this enclosure being ventilated? The additional heat source in the enclosure does not affect the compressor capacity as long as you have the enclosure adequately ventilated.

 

[itsmoked]

If you're concerned about the heat in the compressor box then you can leave the aluminum block uninsulated and it will absorb the heat radiated from the compressor and the "additional" box.

As for capacity it's just as lukai describes. Greatest heat load and highest water-dump temperature. Compressors only come in discrete step sizes. From the load-dump calculation you will need the Coefficient of Performance (COP) of the compressor and then you can select one. Typically you'll miss the standard discrete available offerings by some amount. If you're over one a little go the next larger. If you're right on one you have to make a judgement call of staying with it or going larger.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[ConfusedEng]

Thanks for the replies,

So the other heat sources in the box aren't very important important? the water to be cooled requires 750 Watts based on the quantity & flow rate required, and the compressor is rated to 800W cooling capacity and COP is 2.5 so as long as the water from the water tanks is kept at a constant temp the compressor should manage the load ok?

The enclosure is not very well ventilated, there are a few small holes but it has no forced air convection, adding more ventilation is not it is not really possible. With that in mind should I then be including the additional heat sources into the work required by the compressor? considering the Al block is quite well insulated it will not absorb the surrounding heat very quickly

I will set it up and run some tests to see if it works in prctice, thanks.

 

[IRstuff]

Those numbers seem to be a bit marginal; any flow restriction like fouling is going to mess with the performance

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