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Thread: New idea for higher efficiency
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19-02-2006, 03:14 PM #1
New idea for higher efficiency
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.
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20-02-2006, 05:10 AM #2
Re: New idea for higher efficiency
Originally Posted by star882
rgds
LCI hear...I forget;I see...I remember;I do...I understand
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21-02-2006, 09:09 PM #3
Re: New idea for higher efficiency
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.It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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21-02-2006, 11:59 PM #4
Re: New idea for higher efficiency
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.Last edited by Josip; 22-02-2006 at 12:28 AM.
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22-02-2006, 12:32 AM #5
Re: New idea for higher efficiency
Originally Posted by Peter_1
From the viewpoint of the motor, it is better for it to run since the motor is being cooled (either by refrigerant or air).
Originally Posted by Josip
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.
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22-02-2006, 03:24 AM #6
Re: New idea for higher efficiency
Originally Posted by Josip
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.
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22-02-2006, 05:00 AM #7
Re: New idea for higher efficiency
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...
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22-02-2006, 04:22 PM #8
Re: New idea for higher efficiency
Originally Posted by lin
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.
Originally Posted by Josip
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.Last edited by US Iceman; 22-02-2006 at 04:23 PM. Reason: text editing
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