procedure for Pull down calculation in compressor pakages

[f4760249]

Dear all,

Any body can tellm me the methods's of calculating pull down conditions while selecting compressors (both reci/screw).Suggest any related books

[raviperumal]

Hi,
You can try with ASHRAE Handbook, International standards!!

BR/ravi perumal

[US Iceman]

The pull down condition is what it is. The compressor(s) are sized for the normal operating duty, however, if the systems has regular pull down periods you have to make sure the motor is large enough to run the compressor during the higher suction pressures (during pull down).

At higher suction pressure you can overload the compressor motor (and also possibly the suction accumulator).

Another way is to use a crankcase pressure regulator to maintain the suction pressure at some lower pressure until the pull down is completed.

Reciprocating compressors are OK under these conditions as long as the motor capacity is high enough.

The same thing applies to screw compressors too, but these compressors also have a volume ratio problem, which could increase the power input even more.

If your system has this pulldown condition on a regular basis I woudl go with the crankcase pressures regulating valve. It's the safest way to do this.

[SteveDixey]

Dear all,

Any body can tellm me the methods's of calculating pull down conditions while selecting compressors (both reci/screw).Suggest any related books

What type of system is it?

You can use electronic controllers to prevent motor overloads by using current limiting during pull-down, or restrict the pumping capacity by not allowing full loading of compressors until some particular set-point is reached.

Flooded systems may have a time span per degree C temp, say 1 Deg C per minute as a pull-down rate.

Steve

[Andy]

Dear all,

Any body can tellm me the methods's of calculating pull down conditions while selecting compressors (both reci/screw).Suggest any related books

I assume you are talking about freezer and Blast freezer applications. Select you condensers and drive motors at -20 deg c.

Kind Regards Andy

[D.D.KORANNE]

For pulldown conditions, consider evap temp at least 10 degb C higher and select motor, provide crank-case pr regulator or MOP valve. But do not forget to size condenser to match pull down load. Example , if your design needs -35 Deg C evap ,take pull down suction temp at -25 deg C at least.

KORANNE

[TXiceman]

On a recip you can easily hold out a bank or two of cylinders from loading and keep the power down.

On a screw compressor, it is a bit more complicated than just unloading based on current. The Vi basically defines the internal comppression ratio for the compressor. Vi = (Volume at the suction)/(volume at the discharge) or the pressure ration to the k power.

If you have a high suction pressure, the compressor will develope some very high pressures internally before the rotor port opend to the discharge. A variable Vi compressor can take care of this to a point. The manuafcturers provide a maximun suction pressure for each compressr based on compressor Vi. This is a sustained or lengthy run condition to pull a large system down.

We had a system that we had to design for fully unattended start once the operator pushed the start button. The system had a very large volume of glycol and thermal mass (from the piping) to cool. Based on calculted valuse, the compressor would run for two hours with no load on the system and starting at 125 dF glycol, befoer we had the suction down to an acceptable high limit. The unit was installed with a full line size Fisher V-Ball valve and compressor suciton pressure control from a PID loop in the panel. Once the suction pressure was down to safe limits, the valve was driven wide open. We also used R-134a as the refrigerant to prevent over pressuring the flooded evaporaor in this hot stand by state. Compressor loading was also ramped slowly to prevent pulling the evaporator down too quicly and flashing off the liquid which could lift it out of the evapoarator.

If my memoryservesme correctly, the system had two 700 HP Howdens.

kEN