A/C efficiency

[rmarotta]

Can the efficiency of a 3 phase air conditioner be increased by controlling it's compressor speed at times of reduced load? If so, would the addition of a VFD on the compressor, (suitably controlled) accomplish this?
Could there be enough savings to justify it?
Regards,
rmarotta

 

[cbarn24050]

Hi, yes it can. How much you can save depends on so many things it's an unaswerable question. I'm informed that Californians are paying $0.35 per KWh, so it wouldn't take long to payback there.

 

[rmarotta]

I appreciate your response.
Now I'm wondering what method might best be used for control of the VFD according to part-load conditions.
The difference between room temp and the temp called for?
Suction pressure?
Other?
Thanks for your thoughts.
rmarotta

 

[electricpete]

if you don't get responses here, there might be better qualified people to address that aspect in the hvac forum.

 

[jbartos]

Suggestion: Try to obtain the efficiency versus speed dependence for the A/C from the manufacturer. Then, the A/C unit will have to be driven at that speed yielding the maximum efficiency of the A/C unit.

 

[60hzhmmm]

rmarotta: Yes a VFD will work nicely. You will want to control it to head pressure. You need to be a bit careful here.... Dot your eyes and cross your T's. Make sure the oil pressure remains viable. Too slow equals no oil pressure. As the VFD slows things as the expansion valve throttles back, (as setpoint is achived), at some point you need to go no slower so pump-down can be acheived. Scroll type compressors may be better candidates here. Check with manufacurers. Leave the rest of the system thermostats, expansion valves ect. the same.
You haven't stated why you want to do this. If it is purely for energy savings the biggest payback is reclamation of the rejected heat into water, hot water, often lots of hot water! Remember every milliwatt sucked down the wires to the motor/compressor comes pouring out of the condensor along with the energy removed from the cold space!
And a last point. Often if energy savings is the whole point throttling the condensor fans can help too/instead.

 

[rbulsara]

All modern chiller manufacturers (at least in the USA)offer VFD and control packages as standard and any one of them can be of great help.

Also as eletricpete indicated, please post this in mechanical HVAC/R forum, you will get better answer.

 

[rmarotta]

Thanks all for your comments.
I'm an A/C contractor in Florida, and I like to tinker. I thought I'd work on the unit at my home to see if I might increase it's efficiency.
I have a 5HP/230V VFD and a 3HP/3ph compressor available to play with. Since available power is only single phase, I think I'm correct in assuming that the 5HP drive should be adequate.
Now, if I disregard the cost of these components, I just wonder if it would be worth the trouble to try it.
Can I expect to get any noticeable decrease in power consumption?
Regards,
rmarotta

 

[busbar]

In general, the reliability of 3ø versus 1ø hermetic compressors is one reason to implement the scheme—it may be that this is offset by the fast-risetime output would reduce desirability. From an experimental point of view, some would consider the increased performance data available {sometimes through a port} with many VFDs would alone make the venture useful.

 

[60hzhmmm]

rmarotta: If you have room for hot water storage,(like one or two more hotwater heaters), that would give you the biggest bang for the buck. By far!
There is a company that makes hotwater extraction systems(Thermoastor?) that will allow water upto about 200F to be extracted. Essentially the hot case is run thru an exchanger on the way to the standard condensor so when no more hot water can be accomodated you're back to the orginal situation.

http://www.thermastor.com/pdf/TSII-1_III-1.PDF

 

[jbartos]

Suggestion to rmarotta (Electrical) Nov 23, 2003 marked ///\\I'm an A/C contractor in Florida, and I like to tinker. I thought I'd work on the unit at my home to see if I might increase it's efficiency.
I have a 5HP/230V VFD and a 3HP/3ph compressor available to play with. Since available power is only single phase, I think I'm correct in assuming that the 5HP drive should be adequate.
///Single phase versus three phase would not be a good correlation between 5HP and 3HP, since there are very efficient phase converters for AC-DC-AC conversions. 5HP VFD has sufficient capacity to power 3HP motor.\\Now, if I disregard the cost of these components, I just wonder if it would be worth the trouble to try it.
It is more important to find out how the VFD parameters will meet the motor application including load requirements.\\Can I expect to get any noticeable decrease in power consumption?
///It depends on the motor load nature.\\