Which one is true ?

Dear friends,
I had discussion about this matter in a group yesterday. I have a question.

Suppose we have a 1 ph hermetic compressor.

1 - I was telling when suction pressure and discharge pressure are near to each other it means compression ratio is near to 1 and compressor will start better and easier (We have this matter at air-conditioning system compressor).

2 - Others insist on this matter when suction pressure is low, For example when we pump down compressor before off cycle, Compressor will start easier because compressor has not high load on suction and cylinder while wants to start.

I know while suction pressure decreases, Compressor load and working ampere would decreases, It is clear. But what about starting time ? In which case compressor would start easier ?

Which one is true ?

Sincerely yours.

2 Is untrue.

It is the difference between the suction and discharge pressures that affect the power required to start the motor.

That is why we have LST Low Starting Torque and HST High Starting Torque compressors.

This Danfoss document may help....

https://www.google.co.uk/url?sa=t&so...MdjY7X_r4zZvF8

Quote:

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Originally Posted by Brian_UK

2 Is untrue.

It is the difference between the suction and discharge pressures that affect the power required to start the motor.

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Dear Brian_UK,

OK 2 True.

1 - But I saw a compressor that works in this shape :
There was a solenoid valve between discharge and suction pipe. Before start this solenoid valve opens route between suction and discharge pipe, Compressor starts and after few second this solenoid valve close this route. This solenoid valve for few second just wants to equalize these two pressures between suction and discharge.

2 – Please find my photo at below, There are some lines that I mentioned with yellow color form SUPCO company catalogue.

Sincerely yours.



Sorry Brian_UK , I thought that you told me item 2 is true !!!! sorry

Attachment 15116

Quote:

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Originally Posted by Brian_UK

2 Is untrue.

It is the difference between the suction and discharge pressures that affect the power required to start the motor.

That is why we have LST Low Starting Torque and HST High Starting Torque compressors.

This Danfoss document may help....

https://www.google.co.uk/url?sa=t&so...MdjY7X_r4zZvF8

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Thanks a lot

I was referring to piston compressors. :)

The old prestcold r bodies 5 hp and up had a valve if I am not mistaken man they ran like a sewing machine if treated right!

Mortezania
You can get Txvs which have an internal bleed through the orifice to help the system pressures equalise quickly . The bleed rate is only a small percentage of the main orifice so that the superheat control still operates correctly

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Originally Posted by Glenn Moore

Mortezania
You can get Txvs which have an internal bleed through the orifice to help the system pressures equalise quickly . The bleed rate is only a small percentage of the main orifice so that the superheat control still operates correctly

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Dear Glenn Moore,

Thanks for your advice. Do you have any documents or photo form this type of TXV ?

Sincerely yours.

We have in the past pointed you towards libraries containing the information you seek.

Please make use of these resources.

http://sporlanonline.com/literature-...nal-materials/

Hi, A.Mortezania :)

Quote:

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Originally Posted by A.Mortezania

Dear friends,
I had discussion about this matter in a group yesterday. I have a question.

Suppose we have a 1 ph hermetic compressor.

1 - I was telling when suction pressure and discharge pressure are near to each other it means compression ratio is near to 1 and compressor will start better and easier (We have this matter at air-conditioning system compressor).

2 - Others insist on this matter when suction pressure is low, For example when we pump down compressor before off cycle, Compressor will start easier because compressor has not high load on suction and cylinder while wants to start.

I know while suction pressure decreases, Compressor load and working ampere would decreases, It is clear. But what about starting time ? In which case compressor would start easier ?

Which one is true ?

Sincerely yours.

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In your first post statement no. 1 is correct ... compressor is going to start easier ... when both pressures are equalized i.e. close each other .... valid for any type of compressor ....

Second statement is wrong .... but pump down is another way to equalize suction and discharge pressure within compressor ... compressor can start easier because there is no high load on discharge as a consequence of low pressure on suction ... amperes of compressor depend only on discharge pressure ... i.e. mass of refrigerant to move ... in case of low suction pressure amount of work is smaller due to low mass of refrigerant to move from suction to discharge ...

Quote:

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Originally Posted by A.Mortezania

Dear Brian_UK,

OK 2 True.

1 - But I saw a compressor that works in this shape :
There was a solenoid valve between discharge and suction pipe. Before start this solenoid valve opens route between suction and discharge pipe, Compressor starts and after few second this solenoid valve close this route. This solenoid valve for few second just wants to equalize these two pressures between suction and discharge.

2 – Please find my photo at below, There are some lines that I mentioned with yellow color form SUPCO company catalogue.

Sincerely yours.



Sorry Brian_UK , I thought that you told me item 2 is true !!!! sorry

Attachment 15116

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In your second post statement 1 - with by-pass valve between discharge and suction can help for easier start .... this valve can be open for some short time ... 5-10 seconds max otherwise there is possibility to have too high suction super-heat and consequently high discharge temperature ... leading to alarm and shutdown ... this is generally installed on screws, because industrial piston compressors can be started almost unloaded what is not case with screws ... can be installed also on hermetic piston and scrolls ...

Statement 2 is kind of "snake water" - this can be of some help, but in general coming into this position (hard start) must be solved during project design phase ... otherwise no help i.e. any solution will cost a lot and definitely not good ...


Best regards, Josip :)

Quote:

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Originally Posted by Josip

Hi, A.Mortezania :)



In your first post statement no. 1 is correct ... compressor is going to start easier ... when both pressures are equalized i.e. close each other .... valid for any type of compressor ....

Second statement is wrong .... but pump down is another way to equalize suction and discharge pressure within compressor ... compressor can start easier because there is no high load on discharge as a consequence of low pressure on suction ... amperes of compressor depend only on discharge pressure ... i.e. mass of refrigerant to move ... in case of low suction pressure amount of work is smaller due to low mass of refrigerant to move from suction to discharge ...







In your second post statement 1 - with by-pass valve between discharge and suction can help for easier start .... this valve can be open for some short time ... 5-10 seconds max otherwise there is possibility to have too high suction super-heat and consequently high discharge temperature ... leading to alarm and shutdown ... this is generally installed on screws, because industrial piston compressors can be started almost unloaded what is not case with screws ... can be installed also on hermetic piston and scrolls ...

Statement 2 is kind of "snake water" - this can be of some help, but in general coming into this position (hard start) must be solved during project design phase ... otherwise no help i.e. any solution will cost a lot and definitely not good ...


Best regards, Josip :)

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Thanks Josip