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star882
19-02-2006, 04:14 PM
Isn't it possible to design a compressor that can be started and stopped very frequently for capacity control? The motor and starting components will be designed for frequent starting (even with a pressure difference), and a pair of SCRs or a triac will be used to switch it on and off. There'll be a separate oil pump that runs whenever the unit is in use, to reduce wear on the bearings.
It'll switch on and off with a total period of 40 seconds or so.

I see my idea as a cheaper alternative to VFDs. In fact, about the only changes that must be made to a normal compressor are the motor, possibly the bearings, and the addition of a separate oil pump motor.

Lc_shi
20-02-2006, 06:10 AM
Isn't it possible to design a compressor that can be started and stopped very frequently for capacity control? The motor and starting components will be designed for frequent starting (even with a pressure difference), and a pair of SCRs or a triac will be used to switch it on and off. There'll be a separate oil pump that runs whenever the unit is in use, to reduce wear on the bearings.
It'll switch on and off with a total period of 40 seconds or so.

I see my idea as a cheaper alternative to VFDs. In fact, about the only changes that must be made to a normal compressor are the motor, possibly the bearings, and the addition of a separate oil pump motor.
It sounds similar to copeland digital scroll. Maybe you can check copeland web at first.

rgds
LC:)

Peter_1
21-02-2006, 10:09 PM
Isn't such a good idea: starting something from standstill requires a lot of energy, a lot of stress on the bearings, bad lubrication at startup, excess heat of teh starting will not be reemoved sufficiently...

What you can do is add a controlled HP injection (puls/pause- in the LP and take care that the suction gasses don't become too hot.

Josip
22-02-2006, 12:59 AM
Maybe with reciprocating compressors with electromagnetic clutch. Until now it seeems better to run compressor unloaded totaly (for some time) than to stop it and start in very short time.

Minimum speed for screws are about 1500 rpm otherwise you do not have oil sealing effect, and no work.To obtain oil pressure you need up to 30 sec from standstill. How to start and stop big mass of rotors in very short pulses.

But most important thing, we not need to control capacity in that way. Change of temp or pressure must not be too fast, becuse for heat exchange we need some time. Cooling proces is not hunting but with such control will be.

US Iceman
22-02-2006, 01:32 AM
...excess heat of the starting will not be removed sufficiently...

A very good point. This is the reason why large motors have a certain limit on hot starts per hour. The motors have to run some time for cooling the stator back down.

From the viewpoint of the motor, it is better for it to run since the motor is being cooled (either by refrigerant or air).


Until now it seems better to run compressor unloaded totally (for some time) than to stop it and start in very short time.

I completely agree with this.

Shutting the motor off in quick intervals would allow the motor to retain residual heat which could damage the motor windings.

The electrical energy used in repetitive starting could also be higher due to the frequent generation of starting torque.

A VFD would be much better suited for controlling capacity reduction.

lin
22-02-2006, 04:24 AM
Minimum speed for screws are about 1500 rpm otherwise you do not have oil sealing effect, and no work.To obtain oil pressure you need up to 30 sec from standstill. How to start and stop big mass of rotors in very short pulses.

Carrier 23XRV can run at 15% capacity with vsd on screw compressor. but that triple-rotor screw has synchronized gear to guarantee no rotor contact.

I am curious how low speed related to normal operation on twin-screw compressor would be safe. some varible spped screw air compressor can run at 25% capactiy, but havn't seen many cases on refrigeration.

star882
22-02-2006, 06:00 AM
Slightly better idea: two compressors of different capacities in one case, each rated to start under pressure. They'll be independently switched on and off to match varying load conditions, providing 3 levels in addition to completely off.

I also remember about a simple circuit that uses a triac to slow down a motor (not dimmer style; it conducted one half cycle and skipped one cycle to conduct the next half cycle of opposite polarity, etc.). However, it would run the motor at about 1/3 speed - too slow for most compressors. The pulses would probably make the motor very noisy at the least...

US Iceman
22-02-2006, 05:22 PM
I am curious how low speed related to normal operation on twin-screw compressor would be safe. some variable speed screw air compressor can run at 25% capacity, but haven't seen many cases on refrigeration.

It is completely safe, if proper precautions are used.

Using a VFD on a screw compressor provides very good part load performance. If you leave the slide valve fully loaded and reduce the rotational speed you can expect almost linear part load performance. This is much better than the non-linear performance with slide valve modulation at constant speed.

At approximately 50% speed reduction the rotor tip speed is reduced sufficiently which allows compressed gas to slip from one thread at higher pressure into another thread at lower pressure. In effect, this blow-by gas decreases the pumping capacity of the compressor.


Minimum speed for screws are about 1500 rpm otherwise you do not have oil sealing effect, and no work

I agree 100%.

One thing we have to be careful in describing is the percent capacity and slide valve position. These are different. At 0% slide valve position (fully unloaded) the compressor can still be pumping 10-20% of the rated volume.