HGBP valve application

[Lc_shi]

Hi Sirs,
Hot gas bypass valve are used in capacity control and evap pressure maintaning and defrost in DX coil. I am not sure how about the effect to use in heat pump capacity adjustment at part load. Who can give some comment? thx

regards
LC

[US Iceman]

Hi LC,

Hot gas bypass used for capacity control is provided to maintain a constant evaporator pressure when the system load decreases.

If the compressor does not have an unloading mechanism (reduced volume flow capability), the hot gas bypass allows the evaporator temperature to remain constant. This also provides a constant volume flow, at the appropriate evaporating temperature, to the compressor.

When the system load decreases (less cooling or heating required) without a hot gas bypass valve or capacity control, the evaporating temperature will be reduced, since the compressor is still pumping the same volume (m^3/min).

If a hot gas bypass was used on a reverse cycle heat pump, what could be gained? The hot gas bypass would only false load the evaporator and the compressor.

However, this would also reduce the volume of gas going to the condenser, which could reduce the total heat rejected to the space (that you want to warm).

I would prefer to see an inverter used for the compressor capacity control. As the compressor speed is reduced, the volume of gas pumped is also reduced. The lower mass flow could contribute to an increase in heat exchanger effectiveness, since more surface area is available.

I have not done this before, so this is an estimation of what I think would happen. There is probably a lot more to consider for this topic, but this is my first guess.

Regards,
US Iceman

[chemi-cool]

I want to believe, that in the near future, we will see compressors for refrigeration with speed control.

It will defiantly will make the change.


Isn't it about time, that we should have a break through?
Even a small one?

Chemi

[US Iceman]

Copeland will allow the use of inverters on their semi-hermetic compressors. I have to think Carlyle will also.

For open drive compressors (screw or reciprocating) we have been doing this for some time here in the US. Some of the manufacturers are beginning to provide factory matched inverters for the motors.

As long as the application concerns are accounted for properly I am not aware of any reason why we cannot start doing more of this.

[Lc_shi]

Both experts have given good ideas-.thx.

I've planned to apply VFD compressor(HItachi) for my designed heat pump,but denied by top mangement for the VFD tech is new to us and there is some uncertainty which may cause delay. So the HGBP is brought up. I don't like to use HGBP in AC for it can't reduce the compressor energy consuming but only keep compresor running long time at part load.
Use inverter for normal compressor motor may have some problems, low frequency or high frequency can cause motor overheating. I think it need specially designed motor to match inverter. Now Hitachi VFD compressor f is from 20Hz to 115Hz.

Iceman，
what frequency range for the open screw compressor you mentioned?


regards
LC

[US Iceman]

LC,

The minimum speed I have seen is about 30 Hz, with the slide valve fully loaded. When you get below 30 Hz the rotor tip speed begins to reduce drastically and the volumetric efficiency begins to decrease.

Between 30 Hz and 60 Hz, the part load performance is approximately linear. If the electrical system is 50 Hz, I would expect the performance to decrease at something greater than 25 Hz.

For open drive reciprocating compressors, the part load performance is linear through the majority of the speed range. 20 Hz on a 60 Hz power grid is about the minimum speed, without an external oil pump.

Have you investigated the Danfoss hermetic compressors for your heat pump design? They have factory matched inverters, so Danfoss is taking the responsibility for the drive and motor selections.

Try this link for information.

http://www.ra.danfoss.com/ra/Products/ProductCatalogue.asp?BA=&Division=CC&HL=2&HLID=1000&AppID={c2a95dac-5e01-47df-92cd-5972d837cf1a}&selectedTab=LI#

I tried to copy this from the website. If the link does not work, try the Danfoss compressor area under refrigeration. Look for variable speed application guidelines for the Performer series scroll compressor.

Hope this helps you.

Regards,
US Iceman