Two speed AC compressor

[sailoday280]

We are replacing a split AC system (25 yrs old) for the lobby of our condominium bldg. Capacity is approx. 7.5 tons.
I asked the hired engineer to consider a 2 speed AC motor or 2 stage compressor in his specification so that we could then evaluate the higher initial cost vs long term savings because of increased efficiency.

The reply of the engineer is:
Effectively, two speed compressors are not available on the units of this type. The reason is because varying the refrigerant flow rate (a low and a high speed for the compressor) means that special refrigerant piping has to be installed to assure lubricant return to the compressor.

Does the reply make sense?

 

[berkshire]

Sail Oday,
Is this a ductless split system?
B.E.

You are judged not by what you know, but by what you can do.

 

[Compositepro]

Regardless, the reply does make sense. Lubricating oil is entrained in the refrigerant flow as a part of normal operations. The refrigerant flow velocity has to be with a certain range so you do not get slugs of oil accumulating in the piping.

 

[sailoday280]

Berkshire
The compressor is located about 150 feet from the bldg. The remaining split part of the system is below the lobby from where a few ducts deliver the cooled air.

 

[berkshire]

Sail Oday,
Ok so it is not a ductless system, and if the expansion coil is below the compressor, or if you have a vertical section of return line, then the gas line has to have a velocity high enough to carry the oil back to the compressor. If it does not, you will have all kinds of problems, including an evaporator core filled with oil.
B.E.

You are judged not by what you know, but by what you can do.

 

[sailoday280]

Berkshire
With the outside compressor at ground level and the split system evaporator below the ground level lobby and a small section of vertical return from evap, does that rule out a commercial 2 speed compressor?

 

[MintJulep]

Well, it's sort of a BS answer in my view.

There is nothing "special" about the piping associated with a dual-speed compressor.

Refrigerant piping needs to be sized correctly to ensure that oil return happens.

It needs to be sized correctly for a single-speed compressor. It needs to be sized correctly for a dual-speed compressor. It needs to be sized correctly for a variable speed compressor.

It especially needs to be sized correctly for a long run like yours.

Now, it may very well be true in this specific case - with the constraints of unit size and distance and elevation - that there may be no solution that will work correctly.

 

[cry22]

Your warranty will go out of wack the minute you install against manufacturer recommendations.
Assuming you can find one that will make it for you.

 

[berkshire]

SailOday,
The thing that your Engineer has to figure for you, is whether, or not, at its lowest speed setting, your system has sufficient transport velocity in the return line to bring the oil back to the compressor. On some systems a Hot gas bypass or oil return pump is used, but on a 7 to 8 ton unit which is what you said you have, it is not very often done.

You need to find an engineer you can work with, bearing in mind that you need a person with the, correct answer, for your problem , not the one you want to hear.
B.E.






You are judged not by what you know, but by what you can do.

 

[ozmosis]

Looking at it from a VFD manufacturers perspective.
We develop specialised drives for refrigeration applications, including compressors. Lubrication is an important aspect and has necessitated a function to be developed whereby the drive can be configured to activate, on a timed basis, a 'full speed operation for a short period of time' (to basically recirculate the oil) if the drive is running at low speeds for a continual basis. All of this is included in the drive. If you didn't have this in the drive, you would need something external to activate it.
So, it is not untrue that running at low speeds will create lubrication issues but, this is only part of the story.
As MintJulep points out, the whole system needs to be evaluated to ensure correct operation. Lubrication is just one of them.

 

[cdxx139]

You are at a high enough tonnage that you should be able to get a 2 speed or modulating compressor. Trane can do the 2 speed and Aaon can do the modulating.

If the manufacturer says it is 2 speed, then they did the engineering to figure out how to get the oil back to the compressor, provided you follow manufacturer's refrigerant line instructions.

knowledge is power