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Old October 11th, 2016, 07:24 AM   #1
g.mccormick
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Heating and cooling commands calculation

I have a room (test cell) that I am going to be controlling the temperature of. Longterm the plan is for the space to be able to control temperature from -40-150F. As of right now, no heat is in the space.

What I have for control loops.

1. Space temperature loop.
target = room temperature setpoint
output/command = Value from -50 to 160 that is use as the input to control loop for heater/cooler air out setpoint.

2. DX Evaporator air out loop.
target = air out setpoint from set by command from space temperature loop.
ouput/command = saturation temperature setpoint for evaporator suction pressure.

3. DX Evaporator suction pressure control loop
target = saturation pressure from temperature command out of previous control loop.
output/command = electronic pressure control valve. 4-20MA

4. DX superheat loop
target = 5F superheat
output/command = electronic expansion valve command. 4-20MA


5. Electric heater loop (Note not implemented yet, no actual heater in space)
target = air out setpoint from set by command from space temperature loop.
ouput/command = command to heater scr



Note. There are two dx refrigeration circuits, my plan would be to have both have the same setpoints, but stage the circuits on/off to meet demand.

So my question comes to all of this. How best to handle demand???

I need to figure out a way to handle staging on of cooling units and heater so that both refrigeration circuits and/or heater are not running if not needed.
So I need to have a way to come up with a "cooling command" and " heating command".

I was thinking something like this.
Cooling command:
Min = -50
Max = Room setpoint + 5
Command = ((Max - Space_Temp_Command) / ( Max - Min)) * 100

Example: Space setpoint = 70; Space_Temp_Command (from PID loop 1) = 50
Cooling Command = ((75 - 50)/(75 - -50)) * 100 = 20%
At 20% cooling, I would only need to call for 1 stage.


Heating Command:
Min = Room Setpoint - 5
Max = 160
Command = ((Space_Temp_Command - min)/(max-min))*100

Example:
Space setpoint = 70; Space_Temp_Command (from PID loop 1) = 85
Heating Command = ((85 - 65)/(160-65))*100 = 21


The heating/cooling commands can then be used to stage on/off equipment.
ie. Heater enable on when command goes over 7%, off when command drops below 3%.

DX stage 1, on when command goes over 7%, off when command drops below 3%.

DX stage 2, on when command goes over 60%, off when command drops below 40%.


Any thoughts?
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Old October 13th, 2016, 02:22 PM   #2
shooter
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The evaporator can be controlled with only 1 expansion valve control, You will need a room sensor, and a setpoint for it.
and a suction pressure sensor and temperature sensor on same suction line, to be able to control the superheat of the evaporator.
When the room temperature is reached you just put close the expansion valve, thus the flow will be lower.

For the compressor you will need a suction setpoint about 10 K lower as the evaporator.
This should switch the compressors on/off or a bypass valve....
check on codesys building.lib for nice control loops.
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Old October 13th, 2016, 02:23 PM   #3
shooter
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as heater you can use the condenser heat of the process.
or turn the process like with an airconditioner.
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