Screw compressor

Hi everyone!! I am new to this forum and hopes to get the answers of my queries

We have a chiller in our industry having a screw compressor. It has a slide valve and a 4 step capacity control system, 35%, 50%, 75% and 100%.
I was initially assuming that as the capacity will shift from low to high (say 50% to 75%), the low pressure gauge will show an increase in pressure while the high pressure gauge will remain constant.
When experienced on field, as the capacity control system switches to high value (say 50% to 75%) the low pressure goes down and the high pressure goes up.
The theoretical background tell us that as the evaporator temperature drops, the mass flow rate the refrigeration effect and the refrigeration capacity all drops down so how this system is working actually ?
You can check the curves of mass flow , refrigeration effect and capacity from INDUSTRIAL REFRIGERATION HANDBOOK from Stoecker.

Hi everyone!! I am new to this forum and hopes to get the answers of my queries

We have a chiller in our industry having a screw compressor. It has a slide valve and a 4 step capacity control system, 35%, 50%, 75% and 100%.
I was initially assuming that as the capacity will shift from low to high (say 50% to 75%), the low pressure gauge will show an increase in pressure while the high pressure gauge will remain constant.
When experienced on field, as the capacity control system switches to high value (say 50% to 75%) the low pressure goes down and the high pressure goes up.
The theoretical background tell us that as the evaporator temperature drops, the mass flow rate the refrigeration effect and the refrigeration capacity all drops down so how this system is working actually ?
You can check the curves of mass flow , refrigeration effect and capacity from INDUSTRIAL REFRIGERATION HANDBOOK from Stoecker.

Assammakda,
The compressor loaded so is doing more work.

Mass flow has increased, of course suction pressure decreases to find new equilibrium in system.

but the graphs tell that a decrease in suction pressure decreases the mass flow. How can you say that the suction pressure decrease due to increased load ?

If the compressor is fixed speed & capacity mass flow will be lower if suction pressure decreases.

IF compressor has variable capacity by loading or unloading cylinders, or slide valve on screw compressor, then it's normally loaded/unloaded to control a set suction pressure.
As compressor load/unloads capacity changes up or down.

If you had a compressor that you load manually to drive suction pressure down, it will initially have higher mass flow then taper off as suction pressure decreases.

Signs that compressor has higher mass flow are motor amps, increased discharge pressure or more condenser fans running to control pressure.

I agree with your first statement about mass flow. I was thinking the same.

As you said:
'If you had a compressor that you load manually to drive suction pressure down, it will initially have higher mass flow then taper off as suction pressure decreases." This means that mass flow will decrease as the compressor loads ?
This cannot be right as the compressor loads, it will compress more gas as far as i think.
I am trying to find the reason for lowering of suction pressure and increase of discharge pressure as the compressor loads

Quote:

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Originally Posted by azammakda

I agree with your first statement about mass flow. I was thinking the same.

As you said:
'If you had a compressor that you load manually to drive suction pressure down, it will initially have higher mass flow then taper off as suction pressure decreases." This means that mass flow will decrease as the compressor loads ?
This cannot be right as the compressor loads, it will compress more gas as far as i think.
I am trying to find the reason for lowering of suction pressure and increase of discharge pressure as the compressor loads

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If the compressor loads it will pump more gas, but as suction pressure decreases, the gas is not as dense, so mass flow becomes less & less as suction pressure decreases & decreases in this situation.

That's why we unload compressor to maintain a suitable suction pressure for the application. If you run it lower than necessary you will waste power & probably ice up evaporator.
Basically maintain system balance to suit evaporator load.

[QUOTE=RANGER1;287286]If the compressor loads it will pump more gas, but as suction pressure decreases, the gas is not as dense, so mass flow becomes less & less as suction pressure decreases & decreases in this situation.

First u were saying that mass flow will increase on loading the compressor as it will compress more gas. Now you are saying it will decrease and decrease.
please clear Why the suction pressure DECREASES ?



PS: Ignore if you find it newb

Compressor capacity increase from 50% to 75%, means that volume flow of this compressor is increased. This volume increase pool down the suction pressure. Mass flow will increase as well. Capacity(volume) will increase by 50%. At the same time density of the gas decrease but less than 50%. Mass flow will increase because it is equal volume multiply by density. Example. Capacity was 50 units became 75 units. Density was 1 unit became 0.8. Mass flow was 50 x 1=50 became 75 x 0.8=60. Mass flow will increase.

Thanks everyone