Computing 2 stage Condensing Unit Capacity

[w2jo]

Lets say that I have a 48000 btu/h Condensing unit. In HIGH capacity mode, the inlet temp is (for example) 75F and the outlet air temp is 50F. Lets assume the capacity is actually 48,000btu/h for this case.

Then If, in LOW capacity mode with the same air flow the outlet air temperature is 57F, does this mean that the approximate capacity in LOW mode is something like

(25/48000) = (18/X) where X comes out to be 34,560 btu/h

Or is it more complicated than that.

Can someone give me a bit of education please?

Thanks
joe

 

[MintJulep]

Just a bit more complicated.

Firstly, you ask about a condensing unit, but your temperatures and the a temperature drop from in to out suggest that you actually want to ask about an evaporator.

Second, just because the nameplate says 48,000 BTU/hr does not mean that the system will deliver a capacity of 48,000 BTU/hr at the conditions it is operating at. The nameplate rating was determined at a very specific set of conditions that should be (but most likely aren't) listed.

The capacity at any given instant is a function of the air entering and exiting conditions of the evaporator and of the condenser, and the actual air flow through both.

"High" and "Low" modes are modes, not capacities. The capacity, as noted above, is a function of many things. A mode most likely defines the positions of some valves, maybe the speed of a motor, and maybe some other stuff.

So, assuming that you actually do mean to ask about an evaporator capacity, to determine the delivered capacity you need to know that mass flow rate of air, and the air entering and exiting conditions (dry bulb temp and RH, or wet bulb, or dew point). Then you can look up the enthalpy at those two conditions, and calculate the delivered capacity.

 

[w2jo]

Thanks.. I understand what you are saying.. Yes.. Actually it is BOTH the evaporator and the condensing unit that contribute to "capacity". But (due to my simple thinking), I reasoned that since the capacity control on this system involves a solenoid valve in the condensing unit which bypasses part of the scroll compressor that it was the compressor displacement that was controlling the capacity and that the evaporator and expansion valve were doing their best to keep the evaporator full of boiling Puron regardless of how much was pumping. That is why I referred to CU capacity. Sorry if my nomenclature is a bit "off". You know how EEs are when they start trying to understand ME "stuff"!

As an added bit, the RH in is about 55% at the 75F inlet temp. The Outside Air Temp was about 90F when I took these measurements and that should be in the range where Carrier made their capacity specifications.

I do not have a way to measure the RH out or the actual air flow rate (CFM). My goal is to get a "rule of thumb" for estimating the capacity difference by "assuming" that Carrier got the 48000btu/h (full capacity) reasonably accurate. This so I will be able to judge when a dual capacity system is operating properly. What do you guys use as a criteria when you are doing a system inspection?

I am not after laboratory accuracy, I just want to correlate what Carrier says is 100% vs 66% "capacity" with exit air temp at a constant air volume so as to try and understand how to tell when a 2 stage HVAC cooling system is operating "nominally OK".

In this case, 66% of 48,000btu/h is 31,600btuh and I am getting the above calculation of 34560. Is there a way to "estimate more closely" or should I be happy with this level of "inaccuracy".

Is there a reference for this sort of calculation for HVAC gear when you really do not know the mass flow rate at the input but you know it is a constant? Please feel free to tell me that I should have learned all this in a Thermodynamics class.

Thanks!

 

[imok2]

A mollier chart for the refrigerant you are using and taking the proper readings will tell you what the total system is doing at that particular moment in time.

 

[w2jo]

Thanks.. I would not know where to begin with a Mollier Chart.