Bore & stroke ratio of compressor

[nambiandy]

Hi everyone ,I am Anand a new member from India.

I wish to understand,how revelant is bore & stroke ratio for negative temperature application with feron system.Is it advisable to use a compressor with equal bore& stroke ratio for -35 DegC room temperature .


I had gone through certain compressor selection of Bitzer and copeland and found that few model(Discuss model) offered by copeland has equal bore & stroke ratio.

I was told by a known person that ,when the temperature in the room attains -35deg C the suction pressure at that point is quite low and the compressor does a job of compressing it to higher pressure , and at this point the compressed gas remaining in the clearence volume is at high pressure , to overcome that pressure by bring it below the suction pressure of refrigerant which at very low pressure, longer stroke will help in overcome that pressure.

how far do you think this statement is true??

BR

Anand

[Brian_UK]

Welcome to the forum.

The pressure within a cylinder is dependent on the valve spring pressure and not the length of stroke.

A short stroke piston can generate the same pressure as a long stroke although the volume swept will be different.

[nambiandy]

Hi Brain,

Thanks, what is your recomandation to select compressor for negative temperature application (specially for blast freezing).

should we select the compressor considering Bore & stroke or go with the recomdation of manufacturer.

BR,
Anand

[Samarjit Sen]

For selecting refrigeration compressors for any temperature, you do not have to consider the bore and stroke. Please follow the manufacturers specifications and you will get the proper conditions. I have executed a large number of projects down to -45 Deg C Te and never had any problem. Should you require any assitance you may contact me by sending a PM.

[juosim]

Hi ^^
I think so...
That's rihgt

[monkey spanners]

Hi Anand

This is a very interesting question. Personaly i'd go with what model was recomended by the manufacturer or wholesaler as i'm more at the nuts and bolts end of the industry than the applications end.

Just for interest.

A single cylinder compressor of 5cm bore and 5cm stroke displaces 392cm3
With a 3mm piston to head clearance at tdc would leave a volume of 23cm3 of gas at dicharge pressure to expand as the piston travels down the bore on the intake stroke. The presure of this gas would have to drop below the suction pressure before any more gas would be drawn in to the cylinder.

A single cylinder compressor of 4cm bore and 7.8cm stroke displaces 392cm3
With a 3mm piston to head clearance at tdc would leave a volume of 15cm3 of gas at discharge pressure to expand as the piston travels down the bore.

Clearly the smaller bore compressor would start to draw in gas to the cylinder sooner as there is less volume to re-expand. On low temperature applications with low suction pressures this re-expansion volume would form a greater volume of the total cylinder volume and i think would have a greater effect on efficietncy than on a higher temperature application.
With a higher temperature application the bigger inlet and discharge valves the bigger bore could accomidate may be more important.

This is all based on "gut feeling and common sense" so take it ar leave it as you see fit:D

Cheers Jon

[nambiandy]

hi Cheer jon,

I agree with your view point.

problem is to find a compressor of this combination, may be their is a technical problem with having small bore and bigger stoke.

Anand