Cold Room Compressor on VSD

Hi everyone,

I am curious to know if anyone has run a small 3ph compressor on a variable spped drive before and if there are any issues with doing the same.

What i am trying to achieve is an energy efficient way of running a cold room (rear loading off licence Fridge with storage). Currently i have the compressor only running at off peak hours which is fine for this time of year in Ireland. Over the next couple of months this will not be enough too keep the beer cool enough for my clients customers.

I am looking to install a vsd controlled by a PID from an s7 300. Software is not an issue. Any comments or thoughts would be greatly appreciated.

Regards,
Barry

Barry
This question needs a lot more info provided for us to help. I recommend you consult with whoever built the compressor. The motor will not care so much BUT the compressor is limiting factor.

Yes you can drive the compressor's 3 phase motor with a VFD
BUT some considerations
Hermetic or separate compressor?
How (or if ??) is oil pumped to compressor? What is oil pump minimum speed to ensure adequate pressure to ensure bearing cooling and lube?
Centrifugal or piston compressor?
Minimum speed of compressor?

IF no unloader and especially so with piston compressor starting is a big issue. I would definitely use a vector drive to enable high torque starting.

Compressors are always stashed away out of sight and out of mind with little consideration to cooling. The "compressor closet" is the warmest room in a bar. Running the motor at very slow speed (thus little cooling because the fan runs slow) will be an issue.

Dan Bentler

Dan,

Many thank for your quick responce. At present I know the compressor is a hermetic type and the cold room is being controlled between 5 and 8 degC. The evaporator is ceiling mounted in the cold room with internal fan. With the compressor being of this type what other information is necessary?

Barry.

Its not advisable to run a hermetic compressor on a VFD. With a hermetic compressor your oil charge recirculates with your refrigerant and a minimum refrigerant velocity is required to make sure the oil is returned to the compressor. In a well designed system the refrigerant lines are sized to work with the compressor to maintain the required refrigerant velocity. Getting too much oil downline without it returning to the compressor is one of the most common causes of compressor failure.

Refrigeration systems can be engineered to use a VFD on the compressor but they use compressors designed for that purpose - you usually only find them on very large compressors. However VFDs are becoming quite common on the evaporator fans.

barryoc said:

Dan,

Many thank for your quick responce. At present I know the compressor is a hermetic type and the cold room is being controlled between 5 and 8 degC. The evaporator is ceiling mounted in the cold room with internal fan. With the compressor being of this type what other information is necessary? Barry.

Click to expand...

Barry
Re quick response - you bet.

Depending on size of compressor - yes we NEED HP and while you are at it line voltage - for the money invested:
I would put in a water cooled condensor. This will pay in 3 ways
1. With good cold water will reduce head pressure (longer unit life) and
2. reduced head pressure will reduce motor load thus power bill
3. Taverns need hot water to wash dishes etc. Free (after capital cost) hot water source. CAREFUL make sure you match rejected heat from compressor to hot water heating in the SUMMER. The total BTU of your heat sink must be greater than the BTU dumped in the condesnsor. You will more than likely without de superheater get only 95 or so heated water BUT you can also use that for winter heating. SUMMER IS THE MAIN HURDLE

I would give some consideration to running evap and condensor fans (pump on condensor if you go water cool) on VFD depending on size of unit. Off top of head I would not do if motors are less than 1/2 HP - may be too difficult to find small 3 phase motors.

I would drive the compressor motor with VFD after careful consideration and consultation with compressor mfr. Just because it is hermetic does not indicate what kind of compressor is inside the hermetic can. You may need to redo or add controls. One thing I would add is anti short cycle.

IF you decide to change the piping MAKE SURE as much as possiblle it is downhill run from compressor discharge (or expansion valve) back to compressor suction. Oil in lines does no good and it is not cooling and lubing compressor sitting in the lines.

Dan Bentler

Cheers guys,

Have a lot of information i need to gather. This is an existing installation and it maybe to small for what i was considering. I will gather the relevant information and any additional info i can find and with the help of the guys here i can make an informed decision. Many thanks for all the help.

Barry.

A Hermetic compressor on a VFD is done a lot.
do not run below 25% of Normal rpm. due to oilpump, or oildisc. (the oil is swept inside the house by a the rotation of the rotor.
As this is a closed system (small one no real problems will arise.
However it is not smart to do it as:
The fan will run always costing energy that has to be pumped out by the compressor.
The system will not be very efficient as the motor will take about 75% of the load for 50% cap.
With these small types only get it down in the night and set the setpoint higher in the day. (use simple two thermostats on a timer.

As shooter mentions, even if you could do this, I doubt that you would save any significant energy. First, the drive wastes the first 2% of the loads kw so significant savings has to be made up in the operation of the load at slower speeds. Second, in systems like freezers or coolers with large thermal soak times, running on/off is simplest and most efficient because the off period represents the energy saving time. The off period varies with heat loss nicely.

I again say that I would not expect to save any energy with a VFD.