How Does Capacity Control Based On Speed Work For Recip Compressor? 1

[zamakaze]

How does VFD speed control actually work in Reciprocating/Positive Displacement Compressor?

In case of reciprocating compressor, how does capacity control work with speed control. Since we don’t really measure capacity / flow, is the speed control done based on pressure?

For example lets assume a machine designed for 300 NM3/hr of gas with 3 bar suction pressure at full speed. In normal conditions, we get 300 NM3/hr supply from source at 3 bar and compressor happily compresses the gas working at full speed. Now lets say , the flow is reduced to 150 NM3/hr…in this case what will Compressor respond to? Is it the fact that suction pressure starts reducing as the Compressor initially at its full speed wants to move 300 NM3/hr? And as suction pressure reduces, the compressor speed is reduced to maintain same suction pressure which in turn reduces capacity of compressor? Is this correct understanding?

 

[pmover]

you need to decide pressure or flow control, NOT both as primary process variable. Now, it is possible to have some limited control of secondary process variable.

regarding the compressor, some capacity control is done via adding volume to compressor cylinder or deactivating cylinder(s) ends. further capacity control can be done via speed/electric motor.

a P&ID of the system process would be helpful to provide further details.

 

[LittleInch]

What you really need is the operating and control philosophy for your system and how the compressor fits in.

But by the sound of it you are probably correct, i.e. the compressor speed and hence throughput is controlled by inlet /suction pressure.

Otherwise if the flow goes down the compressor still keeps going at full speed and would simply draw more gas out of the inlet system than someone is putting in and depending on how much volume their is on the inlet side, the pressure would fall either very fast or more slowly, but it would fall until it ran out of gas.

But each system is different and you need to look at both the P&ID to see what it says and also the control logic / philosophy.

Or just observe it in action and see what it does!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Controlling a compressor by changing its speed based on the inlet pressure is not "flow control". That is "inlet pressure control".

If compressor speed was varied with a flow rate signal, that would be "flow control", but you say you do not have a flow meter.

But there is another way. Controlling a PD compressor's speed can easily be correlated to flow rate. I assume by "capacity", you mean "flow rate".
A recip compressor with vfd can be used for flow control all by itself.
Here is how that works.
Cylinder Displacement = Cylinder(s) volume x compressor rotational speed
Rotational speed is determined by the vfd setting.
Cylinder volume can be varied somewhat (+/- 10% or so) by adjusting cylinder head clearance volume.
Actual gas flow rate is Cylinder displacement x gas density at inlet conditions.

Controlling flow rate must be calibrated to inlet conditions, as Gas density is dependent on inlet pressure. Therefore the flow rate will vary within the range of inlet pressure conditions. At an inlet pressure of 100 psig, flow will be twice the flow at 50 psig and one half the flow rate at 200 psig, given the same inlet temperature and cylinder rpm. VFD speed setting can be calibrated to flow rate by knowledge of gas density and volume at inlet conditions.

Discharge pressure and temperature are a function of cylinder compression ratio.


2 cylinders
Cylinder volume at beginning of stroke 1 ft3 each =2 ft3 total
Cylinder volume at end of stroke 0.5 ft3 each = 1 ft3 total
Compression ratio is 2 to 1

Compressor speed = 500 rpm
Cylinder displacement is 500 rpm x 2 ft3 / rpm = 1000 ft3/ minute x 60min/h = 60,000 ft3/h

Inlet conditions 15 psia and 60F (Standard conditions)
Since inlet conditions are std, flow rate = 60,000 ft3/hrs
Discharge pressure is 2 comp ratio x 15 psia = 30 psia.

If inlet pressure was 30 psia, flow rate would be 30/15 × 60,000 = 120,000 ft3/hr
Discharge pressure would be 60 psia.




Eistein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[FacEngrPE]

If you are driving a positive displacement compressor from a VFD, remember that the shaft torque is proportional to pressure rise across the compressor - Usually this results in the motor seeing constant torque, unless the discharge pressure is a strong function of flow rate. As indicated above overall system dynamics is important in determining how this component needs to work, and the resulting equipment selections.

I did use 1503-44's trick using a positive displacement device (in this case a screw pump with VFD) to replace a centrifugal pump/flow meter/control valve lineup that was providing unsatisfactory service (Suction was a tank that could be filled so that the pump conditions ranged from DP being in both the direction of flow to the opposite). The VFD did need a breaking resistor.
The system was able to successfully meter a constant flow over the range of DP's as good as the process needed, the control scheme was open loop control of the VFD speed input.

 

[1503-44]

Works well enough for most things, at least those that don't involve custody transfer.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."