Screw Compressors

[frank]

I'm being asked to put some DX cooling onto an Air Handler but there is a problem.

The AHU has an inverter driven fan and responds to a demand on the extract fan which serves a number of Fume extract cabinets.

Basically, as the fume cupboards are put into use, the extract fan ramps up and then down as the fume cupboards stop being used. The fresh air AHU follows suit and there is never a constant air volume.

I've told them that the proper way to do it would be to use a chiller and a 3 port diverting valve but apparently, the roof structure will not support a chiller - so DX it is.

Now, I'm thinking, rather than use a series of small compressors, maybe I could use a small Screw compressor with a slide valve that will respond to, say, a 0-10v DC input.

The AHU has a 2 circuit DX coil fitted but I could pipe these up in parallel.

Daikin use this type of compressor on the new chiller range, just wondered if anyone knew if this type of compressor is readily available and - would this idea work?? (would it respond quick enough to satisfy a rapid change in demand)

[US Iceman]

I have had some experience with a VAV system used for comfort cooling using DX. I don't think this is much different than what your are usggesting Frank.

The system I saw also had screw compressor in it. There were continual problems with the TXV's not controlling well and flooding out the compressors. Not a good situation.:(

The chiller is much better suited to this application, unless you get into using another type of liquid refrigerant feed to the evaporators.

Before I get too excited about other methods, what are the potential capacity ranges and increments you might see with these loads?

[Brian_UK]

Is there any possibility of using inverter driven AC condensing units, one for each coil?

Replace the TXV with an EXV and have fun with the electronics.

[coolments]

Hi Frank, will you be able to get the cooling load into the space when the AHU is in its lowest speed, if so you could install a smaller AHU with fixed motor speed and DX and suck air out of the inlet duct as close to the air on as possible and then push it back in just before it goes into the inverter AHU so both return air and supply air for the DX AHU taps into the return air for the inverter AHU, as long as your DX AHU fan is not more powerful that the inverter AHU fan on its lowest speed you shouldnt have a problem.
This way you dont fight against the inverter AHU's fan and can get the fixed speed you require for fridge side to work, basically cooling the air down before it reaches the Inverter AHU in its own return air duct.
I have installed cooling to a few clean rooms on this principle where large air flow was required for better filtration and control, same priciple might work for you in this aplication.

[Dilip sahasrabu]

Can anybody tell me how to start new thread?

[nike123]

Can anybody tell me how to start new thread?


http://www.refrigeration-engineer.com/forums/faq.php?faq=vb3_board_usage#faq_vb3_forums_threads_posts

[frank]

Thanks chaps.
Coolments
The extract from the labs is via a separate induction fan with inverter control on the motor, whose speed is determined by the amount of fume cupboards in operation.

The minimum air flow is set at 0.8m3/s with no fume cupboards in operation, and this volume can rise to a maximum of 4.9m3/s with all 17 fumes cupboards in use. I would assume that the air volumes in between would be increases of ((4.9 - 0.8)/17).

The make up air via the Air Handler is on full fresh air and the input volumes mirror the extract volumes, (less a negative pressure reduction) and is also dependant upon the number of fume cupboards in operation.

The cooling load at peak conditions would be 90kW giving a dt of 11K.

Now, if we had minimum load conditions (0.8m3s) we would only need 15.7kW. With a twin circuit DX coil installed at present and 2 x 45kW compressors we would be getting a dt of 31.5K - and this would just lead to a frosted coil.

Maybe we could use 2 x 45kW compressors with 2 stages of unloading, say 100%, 70% and 50%.
Although not ideal, with a little bit of reheat to protect the low limit, it just might work.

Seems messy though compared to a screw (and even better, a chiller)

[chemi-cool]

Water Frank,
You are inviting problems with DX.

Try to sort it out with a water chiller and a bank of cold water for high peaks.