Refrigeration condenser selection help
November 9, 2013 12:23 PM   Subscribe

Hi, I'm a college mech eng student and my group has been tasked with designing a small refrigeration/heat recovery system from scratch. We've selected the compressor based on the cooling load and the evap and condensing temps that the professor assigned. Now we're trying to find an off-the-shelf condenser but we're stuck on how to evaluate the ones available to know how well they fit our needs. (details inside)

So the problem is this. We see that the specs provided for condensers show the max heat transfer rate, max pressure and temp, and max flow rate. However, we must not know how to use these for our selection because, based on our total heat rejection and also the temps, pressure, and flow rates predicted by our computer model, it appears that just about any one would work for us (because we're beneath all of their maximums and their heat rates generally always exceed our needs).

How can we use the condenser's specs and our data to choose the best one to fit our needs?


System constraints are:
-max cooling load = 8500 Btu/hr
-condenser needs to be a plate heat exchanger so that the waste heat can be recovered and used to heat a tank of water to near 120 F. The water from the tank is continuously recirculated through the condenser using a pump. The water flow rate can be chosen based on needs and pump cost.
-refrigerant evaporation temp should be near 30 F (for future functionality additions)
-use scroll compressor (for low noise) to be speed controlled in the future
-shoestring budget

The professor approved the compressor so we're working with it.
Spec sheet here: ZS09KAE_TF5.pdf
Webpage here

Info from that compressor's specs sheet:

At 6040 Btu/hr
       Evap temp = 20 F
       Superheat = 45
       Condensing temp = 120 F
       Subcool = 0
At 9640 Btu/hr
       Evap temp = 45 F
       Superheat = 20
       Condensing temp = 130 F
       Subcool = 0

Using the specs for 6040 Btu/hr, our software model of the system produced:

R134a temp out of compressor (and into condenser) = 225 F
R134a temp out of condenser = 118 F
R134a pressure high side = 186 psi
R134a pressure high side = 33 psi
R134a flow rate = .0264 lb/second = .72 lb/min

(The model is based only on refrigeration process equations, R143a properties, and the operating conditions from the compressor spec sheet (so no real heat exchangers, cooling water tank, etc.). Also, for simplification, the model is assuming no pressure losses/friction effects within the system, or heat exchange with the surroundings except for heat in during R134aevaporation and heat out at the condenser.)

How can we identify candidates and choose the best (of the cheapest) one that will fit our needs using this information we have?

(We don't expect the size or pressure drop to be factors in our selection, but cost is one of the biggest.)

We appreciate your help.
posted by atm to Technology (4 answers total)
If they all fit the specs, you pick the cheapest one. Cost optimization.
posted by mr_roboto at 12:33 PM on November 9, 2013

Best answer: To clarify: by cheapest, I mean lowest annual cost of operation + depreciation.
posted by mr_roboto at 12:35 PM on November 9, 2013

Response by poster: Thanks for your reply! The problem is that we're not sure exactly what "fit all the specs" means. I mean, yes, as I said our values are below their maximums. But, I think the biggest question we have is how they arrived at their heat capacity ratings. What conditions need to occur in our system so that we can achieve the given heat rating for a plate heat exchanger?
posted by atm at 2:53 PM on November 9, 2013

Best answer: I'm struggling a bit with the question.
You have vendors / condensers identified right? Why don't you contact the vendor with your data and see if the condenser will meet your duty? If not, they will usually find you a model which will meet the requirements. Also, if a heat exchanger is oversized, you can restrict flow of the cooling liquid (the liquid you're trying to heat in your case).
note - I've done cooling system design, but always steam / water / air systems, no refrigerant. If you want to mefi mail me, we can discuss in more detail.
posted by defcom1 at 7:37 AM on November 11, 2013

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