Hot GAS BYPASS OR VFD IN A SREW COMPRESSOR

In a strawberry frozen plant using a ammonia system. It has one compressor, one evap condenser and a ammonia separator which has two pump to over-feed a helicoidal frozen tunnel.

The Te= -20ºc, Tc= 20ºC

The tunnel normally works at 50% of its capacity so does the compressor and some time the refrigeration load goes to 0% due to the lack of strawberry, and compressor stop

I though to install a hotgas by pass between receiver and compressor to keep it working .

However some friends of mine recommend a Variable Frequency Device (VFD) for that application.

What do you recommend me?

I have used a VFD on a ***** screw compressor and it works great. It offers great capacity control with added benefit of the energy savings of varying the frequency down.

I would go for a VFD, no matter what...

We use them on 99% of all our systems, and it has benefits both in energy saving and a smoother capacity control.

You can promise your customer a reduced energy bill by installing a VFD. Here in Norway, land based factories pay for the peaks, so if they stop and start a huge screw compressor in the off season, it means they will pay for the highest peak of power consumed for that entire period.

With a VFD, they get a much lower peak during starting, since it acts as a softstarter, and the compressor can keep running, instead of starting and stopping, when the plant only needs "maintenance" cooling instead of production capacity.

(and we all know that starting and stopping frequently is bad for any compressor)

I don't know what kind of system you have in place for capacity control, if it's a compressor with slidevalves or ports.

But the way I would do it, be it slide valve or ports is:

Check with the compressor manufacturer what the minimum speed of the compressor can be without loosing compression (to slow speed and you loose compression and the screws bleed backwards).
make a capacity regulation system so that the compressor starts on minimum capacity and ramps up to around 500 rpm over the minimum speed rated by the manufacturer, as demand rises, increase the capacity on the screw compressor until it reaches 100%, then start ramping up the speed on the VFD.

when the capacity is not needed, just reverse it, ramp down the speed, and when the speed is at minimum, decrease the mechanical capacity on the compressor...

this way, when there are no strawberries, you can run the compressor at maybe 1700 rpm and mechanical cap at 10/25% and have enough to maintain temperature in the room.


I have always considered hotgas bypass as biting yourself in the tail and throwing money out the window.

VFD is the best but also depends how much money there is to spend.
You may also need false load from reciever as well!

Other VFD problems you may encounter are vibration at certain frequencies (skip these frequencies).
Hydraulic tubes fracturing on unloader system if it has external tubing(Frick not a problem as internal porting).
Oil compression under very low loads & no gas flow through compressor(need oil management, maybe on oil injection).

As I Understood, if the suction pressure goes down due to the lack of refrigeraction load, the rpm of the compressor goes also down to keep the suction pressure setpoint. The rpm goes to a minimal according with the compressor manufacture. From that minimal rpm we have to close the slide valve if the pressure keeps going down.
The question is: Can the compressor keep running without load , minimal RPM and slide valve closed ?
Gwapa

Quote:

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Originally Posted by gwapa

As I Understood, if the suction pressure goes down due to the lack of refrigeraction load, the rpm of the compressor goes also down to keep the suction pressure setpoint. The rpm goes to a minimal according with the compressor manufacture. From that minimal rpm we have to close the slide valve if the pressure keeps going down.
The question is: Can the compressor keep running without load , minimal RPM and slide valve closed ?
Gwapa

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Yes but oil compression in compressor may be an issue in some cases.

Quote:

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Originally Posted by gwapa

As I Understood, if the suction pressure goes down due to the lack of refrigeraction load, the rpm of the compressor goes also down to keep the suction pressure setpoint. The rpm goes to a minimal according with the compressor manufacture. From that minimal rpm we have to close the slide valve if the pressure keeps going down.
The question is: Can the compressor keep running without load , minimal RPM and slide valve closed ?
Gwapa

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When the suction pressure goes down, first the compressor should unload on the speed, if the suction pressure keeps below the setpoint you should unload the slide valve.

it all depends on the compressor type, but the compressor should be able to work on it's minimum RPM, with the slide valve completely unloaded.

but you need to find out from the compressor manufacturer what is the minimum RPM.

the main problems on screw compressors with VFD is if you go to low on the RPM you don't get any compression and if the compressor is lubricated by differential pressure, you loose oil pressure, or you get to high discharge temperature, For howden compressors I think the limit is 1400 rpm, but I am not sure.

Thanks a lot