Centrifugal R-134a Air Conditioning Unit

[mauner]

Hello all,

I have a 200 ton unit designed for chilled water at 720 gpm and 44F supply.

What happens to the compressor power consumption/efficiency if I increase the flow to 900 gpm but keep the same load and supply temperature?

Simply put, from a kW/ton perspective is it better to have a higher inlet(return) temperature and lower flow? (Assuming UA of the evaporator is constant)

Thanks in advance

 

[KiwiMace]

I think the proposed plan is less efficient within the chiller unit itself. The evaporator is now transferring heat from a cooler fluid incoming at a smaller delta-t to the refrigerant. So less heat transfer driving force with a greater mass-flowrate to cool.

Aside from addressing the other issues relating to pushing 25% excess flow through a chiller, Your system efficiency will also be reduced: You are pumping more water through the system, your end-user equipment delta-t's are reduced, and your system pressure is higher - particularly in your chiller.

 

[HVAC68]

If you pump in more water, the delta T reduces - the chiller is probably designed to a particular capacity and can't give more than that.

Q = m Cp Delta T.

If "m" goes up, Delta T come down.



HVAC68

 

[mauner]

I think my question was misleading so I will re-phrase.

I understand that I have decreased overall efficincy because the chill water (CW) flow and pumping power is greater than the design flow for the given load.

I am trying to build a case to add a heat exchanger with a much greater temperature rise than that normally seen with an HVAC cooling coil.

I have power consumption curves for varying condenser cooling water inlet and CW outlet temperatures. But I don't have any for varying CW inlet temperature.

If the evaporator is designed for an average (CW)temperature of 47.35 F with an exit temperature of 44F what happens to power consumption if the CW inlet (and average) temperature is increased for the same load?

 

[KiwiMace]

I think my original reply addressed this. You are raising your Chilled water return temperature with a given CHW supply temp. Efficiency will be reduced - the heat exchanger has to do the same amount of effort with a reduced delta-t.

Chillers also reduce efficiency for both increased Cond water supply temp and reduced CHW supply temp.