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vinod
12-12-2005, 09:34 AM
Hi,

Anybody know if screw compressor moving reverse rotation,what will happen to the compressor?

Sabroeclaus
12-12-2005, 10:12 AM
Hi.
I only know from a Sabroe comp.that it will damage the rotors
But its depending on how long time it has been running.

Best Regards
Claus

Rfcont
12-12-2005, 10:26 PM
Hi Vinod!
I had the case
chiller didn't have relay of the control of phases
and the compressor has turned in the reverse rotation some minutes
But when i change phases all became OK.:rolleyes:

US Iceman
13-12-2005, 12:13 AM
Hi,

The screw compressors usually have suction and discharge check valves to prevent reverse rotation when the compressor motor is de-energized.

When the motor is stopped, the compressor will run in reverse for a few turns until the discharge pressure is equalized with the suction pressure. This is done to reduce the inrush current during start-up.

Occasional reverse rotation is normal during shutdown. Continued reverse rotation can damage the rotors as Sabroeclaus stated.

The compressors are not built to be run in reverse. The thrust bearings would be on the wrong side of the rotors also.

TXiceman
13-12-2005, 04:02 AM
As US Iceman said, the larger screws have both sucion and discharge check valves to prevent reverse rotation on shut down. In this case, running backwards will pressurize a checked valve and prevent flow. Due to the pressure characteristics of a screw, it will over pressue the suction side and possibly crack the suction housing and damage the thrust bearings as the thrust on them is reversed and the balance piston (if equipped) is operating in reverse.

I have seen a couple of larger 500 plus HP machines, one Howden and one Dunham-Bush (now Hartford compressor) destroyed by reverse rotation. The problemis they are not protected for high pressure on the suction and they will easily exceed 300 PSIG when running against a blocked condition.

If the compressor is still running, you are lucky and still do not know the extent of damage to overloaded bearings for long term operation.

Ken

Lc_shi
13-12-2005, 05:58 AM
Screw compressor 's oil lubrication is normally built by suction and dischange pressure difference,at the start,it's allowed running 20s before the pressure setup, if in reverse rotation,the presssure cant set up, i think it's one of the reason which cause damage.

rgds
LC-:)

US Iceman
13-12-2005, 07:38 PM
Hi LC,

Some compressors develop oil pressure by differential pressure of the system (discharge - suction). In other types the oil pressure is developed by a separate pump. In this case the oil pump discharge pressure is higher than the compressor discharge pressure.

It is hard to make general statements that can be applied to any situation, so anyone reading these statements should be aware that these are general comments.

The concept you mentioned is also one possibility.

US Iceman
16-12-2005, 03:10 AM
If we have an overall compression ratio of 9/1 which is handled in two stages by square root meaning each stage has a compression ratio of 3/1 then the high to low pressure difference on the second stage is 6 while on the low stage it is only 3.

If the overall system pressure ratio is 9/1, then each stage (low & hi-stage) pressure ratio is 3/1. The ratios are a square function and not deductible. It is a 3/1 ratio for each stage.

Obviously, the 3/1 pressure ratio for the low stage compressor will produce much less differential pressure to establish oil pressure. If the difference of the system discharge pressure minus the suction pressure is not sufficient to provide the required oil pressure an oil pump is required for continued operation. I think we are stating the same thing here.

In some cases a pre-lube oil pump is used to establish sufficient oil pressure before starting the compressor.

The net oil pressure can be dependent on the screw compressor manufacturer. Some use discharge pressure minus suction pressure to create oil pressure, others use oil pump discharge pressure above the compressor discharge pressure.

US Iceman
16-12-2005, 03:37 AM
However, with true positive displacement compressors...

A recip compressor is positive displacement compressor also. Just as much as a screw, scroll, or rotary compressor (not a centrifugal).


Screws and scroll take a volume and compress it according to the internal volume ratio and length of rotor enclosed regardless...

Recip compressors operate in a similar manner. A positive volume of gas flows into the cylinders. When the gas is compressed it does so at a specific volume ratio (the pressure ratio of the operating conditions raised to the specific heat ratio of the refrigerant).

A recip compressor is by it's nature, a variable volume ratio compressor. The only over compression that occurs is due to the discharge valve spring rate and the discharge port losses.

Industrial screw compressors normally have two check valves provided; suction and discharge. When the motor is de-energized, the compressor slows down. Since the motor is no longer providing a motive force to spin the rotors, the rotors begin to stall.

As the discharge pressure in the compressor decreases below the discharge line pressure, the discharge check valve closes. At the same time, the internal pressure of the compressor begins to increase higher than the suction pressure, so the suction check valve closes.

Another value of the suction check valve is to reduce the amount of oil blowback into the suction line during shutdown.

It is normal for the rotors to back-spin a little during shutdown. After the compressor comes to a halt, the internal pressures of the compressor is vented (or equalized with suction pressure).

By having the compressor at an equalized pressure before starting, the in-rush current is much less when it is started. This also allows the rotors to spin up faster with "less compression" occurring.

This is also the reason for having the capacity slide valve fully retracted. With the slide valve at 0% slide valve travel, the compressor displacement is much less. Therefore, less displacement and lower differential pressures allow the compressor to spin up faster, hence lower in-rush current.

This may also be minimized by the type of starter used. Full voltage start is the worse for big screw compressors, since the in-rush current can be 700% of locked rotor amps. This is why a wye-delta or solid state starter is additionally useful.

US Iceman
17-12-2005, 02:46 AM
Let's take a look at this...

I'm going to work in IP units.

"find interstage pressure"
P_interstage=(P_cond*P_evap)^0.5

" find differential pressure for hi-stage compressor"
dP_histage=P_cond-P_interstage

" find the pressure ratio for the hi-stage"
PR_histage=P_cond/P_interstage

" find differential pressure for lo-stage compressor"
dP_lostage=P_interstage-P_evap

" find the pressure ratio for the lo-stage"
PR_lostage=P_interstage/P_evap

To use your numbers, I need a 95F condensing temperature (P = 195.9 psia) and a -13.35F evaporating temperature (P=21.77 psia) for the low stage.

Using ammonia this gives,

"find interstage pressure"
P_interstage=(195.9*21.7)^0.5 = 65.3 psia

" find differential pressure for hi-stage compressor"
dP_histage=195.9-65.3 = 130.6

" find the pressure ratio for the hi-stage"
PR_histage=195.9/65.3 = 3

" find differential pressure for lo-stage compressor"
dP_lostage=65.31-21.77 = 43.54 psia

" find the pressure ratio for the lo-stage"
PR_lostage=65.31/21.77 = 3

"find overall pressure ratio"
PR=195.9/21.77 = 9

At this point we have the overall pressure ratio, the individual pressure ratio of each compression stage, and the differential pressure of each compression stage.

This is all we need.

Now trying to explain your comments about 6 and 2"differential"???


but a 6 differential for the high stage and a 2 (not 3) differential for the low stage.

" find differential pressure ratio for hi-stage compressor"
dP_histage=195.9-65.3 = 130.6 psia/21.77 psia = 5.99, say 6

" find differential pressure ratio for lo-stage compressor"
dP_lostage=65.31-21.77 = 43.54 psia/21.77 psia = 2

Basically, this is one way of stating the differential pressure for each compression stage as a ratio of the lo-stage inlet pressure.

I don't know why you had to go to such great lengths to say this. All you need to know are the pressure ratios and differential pressures. It was an interesting exercise though. So yes, I have to agree with you, the numbers are 6 & 2 for this example.

You should have been a little more clear in your explanation as your point was not well formed.