Horsepower vs Suction Curve - Natural Gas Compressors

[sowetoddid]

I am trying to understand the physical events that are occurring within a compressor cylinder. Maybe someone can help.

ISSUE 1:
Maintaining a constant discharge of 500psi and varying the suction pressure between 100 and 450psi, I calculated the required horsepower. The volume was always fixed at 10MMSCFD. The required horsepower consistently decreased as the compression ratio decreased.

ISSUE 2:
Maintaining a constant discharge of 950psi and varying the suction pressure between 275 and 775psi, I calculated the required horsepower. The volume was allowed to vary. The maximum volume continued to decrease as the suction pressure increased. This makes sense to me because more particles are entering the cylinder on each stroke.

The horsepower continued to increase until the suction reached approximately 480psi. At that point the horsepower started to recede. The overall horsepower versus suction pressure curve followed a bell-shaped pattern.

Can anyone explain what is happening within the cylinder that causes the horsepower to at first increase and the decrease? Also, why does this not apply when the volume is fixed?

Thanks in advance for you input.

Luke

 

[zdas04]

The first "issue" is just the way things work - higher suction pressure = fewer ratios, therefore at the same discharge pressure and flow rate, the work decreases.

I can't make any sense out of "issue 2". You've got too many degrees of freedom. I can see holding hp constant and letting volume change with increasing suction, but letting both float is indeterminant.

The bell curve you talk about comes from the P(suction)*(ratio^((k-1)/k)) term in the adiabatic hp equation. The ratio term decreases as a smooth curve while the P(suction) term is increasing as a straight line. The result is a bell curve.

The Cooper-Bessemer equation does not show this bell curve, but the Ariel program does.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[CRG]

Issue 1. Are you comparing the horsepower requirements for a compressor with different compression ratios and different displacements with a constant mass flow?

Issue 2. Are you comparing the horsepower requirements for a compressor with the same displacement and changing the compression ratio resulting in a variable mass flow? At a glance, it looks like you are not describing a volume of flow at some standard conditions in this case.

 

[Warpspeed]

I am by no means an expert in this field, but will stick my neck out a bit here.

As suction pressure falls, mass flow will also fall because less molecules per stroke enter the pump just as you say, but pumping power still reduces even though the pressure differential has gone up.

When suction pressure rises closer to discharge pressure, the mass flow per cycle increases, but only up to a point. At some stage there may be flow limiting due to the restriction of valves and ports.

I had this exact same problem with a homemade air compressor. As compressor crank Rpm were increased by changing the pulley ratio, the drive motor current increased, but only up to some point. Beyond that I could not load the drive motor sufficiently no matter how fast I ran the compressor crank.

At that stage I started doing some modifications to the valve plate and ports to allow a higher mass flow. And it worked. In stages I increased the motor drive power without changing crank Rpm or pressures.

I have since learned that refrigeration compressors are deliberately designed to limit drive horsepower so that the motor can never be significantly overloaded at any operating condition. Compression ratio and valve sizes can be juggled to keep drive horsepower reasonably flat over a very wide range of suction and discharge pressures.

That is not going to improve compressor peak efficiency, but it might prevent motor overload during sustained abnormal operation.

I suspect your compressor may reach some sort of flow limiting condition that prevents any further flow increase with increasing suction pressure. The flow through the valves may actually go sonic. From there the drive power will fall if suction pressure increases even more.

 

[ccw]

Hi sowetodid,
For what it is worth, if you keep going up with the suction pressure, passing 950 and beyond, the input HP will go through zero and maybe negative as the pump turns into a motor.