Mycom WB runing unloaded for extended periods

[mattmechanic]

hey all
thanks for taking the time to read this!
just wondering what peoples thoughts are on having compressor cylinders running unloaded for extended periods lets say 50% and 80% of the run hours on a machine for example due to it being way way over sized for the job ???

i was always told they shouldn't run unloaded for more then like 30 odd min with out loaded up because they can burn out ? glaze up ect?

just wanting to see what you guys thoughts

thanks

[mbc]

Hi

it depends on what kind of compressor your are using
in open type = you can run ( with DX system needs Elec. expansion valve)
in semi hermetic usually manufacture say how to use it

Also in open type screw and semi hermetic screw one that is different

[RANGER1]

On Mycom not ideal, but does not matter if that’s what you have to do.
Oil may build up in that head after awhile.
Water cooling helps keep cool if on ammonia which runs hotter.
As long as 2 cylinders pumping keep on running ok
Max discharge temp trip should be set 140 deg C
If belt drive you could slow down to minimum speed 800 rpm.
Can’t imagine VSD drive could be an option due to price.

[PaulZ]

Hi Matt
If the compressor is so oversized I would do as Ranger suggests and slow it down to start with. If the compressor is controlled by a PLC you can program it to load a different pair of cylinders each time it loads. I have done this on a few compressors where the load drops off for extended periods and there is no small compressor available.
The main problem with running cylinders unloaded for a long time is the top of the cylinder, piston and valves get fairly hot due to re-compression and no cool gas passing through, not good.
Regards
Paul

[mattmechanic]

hey guys thanks for the reply's
yes on ammonia with water cooled heads
and i already have changed pulleys and running at minimum speed
its just way to big its a slap together cold store out of second hand stuff that we picked up the service on and its all out wack nothing has been sized properly

changing the PLC to load different sets is a good alternative to try and extend its life

[chilliwilly]

How about using one or more transformers to reduce the voltage and keep all the barrels online.If its a calafudged system made out of sha* ends, you could set up pressure switches to drive contactors connected to the secondary trappings.

I have done this on DX systems when I have experimented in modernising butchers shops requirements expanding their displays and storage, using single and three phase dual and single hermetic compressors not matched in size. The loads are low and mid temperature, one compressor is connected to a transformer with 5 secondaries offering 5 speeds, switched by 5 pressure switches set approx 3 - 5 psi cut in stages, which power timers that switch contactors.

The condenser has twin coils isolated from the discharge by a sol valve controlled by a hp switch, and each fan is switched first before the next stage contactor is switched maintaining an optimum head. Each timer are set to an optimum value and its contacts are in series with the next stage pressure switch contacts and offers a delay in switching the next stage compressor speed, and allows the fans to compensate for low ambience.

The other compressor is DOL, and when the variable speed is switched to full voltage, it comes online and the variable speed compressor switches off until a 6th pressure switch set at the highest value switches it DOL.

[RANGER1]

hey guys thanks for the reply's
yes on ammonia with water cooled heads
and i already have changed pulleys and running at minimum speed
its just way to big its a slap together cold store out of second hand stuff that we picked up the service on and its all out wack nothing has been sized properly



changing the PLC to load different sets is a good alternative to try and extend its life

Matt,
You could consider slowing compressor down to 650rpm & use low speed oil pump, if it was going to help.
Pump has bigger gears to deliver required oil pressure at 650rpm.
Interestingly Mycom do not recommend it anymore for some reason, but still sell oil pump.
Machine would last forever at 650 to 800 rpm. (800rpm minimum speed for standard oil pump).
How many cylinders does your machine have?
How many cylinders do you think it requires winter & summer operation?