Revolutions per Minute

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Revolutions per Minute

Basic Thermodynamics for Reciprocating Compressors

P-V Diagram

What is going on inside a typical reciprocating compressor cylinder?


The above diagram is called a Pressure / Volume Diagram or P-V Diagram and demonstrates the ideal, basic dynamics inside a cylinder with the crank making 1 revolution. This cycle represents one end of the piston or a single acting piston. The same basic thing happens on the other side or crank side of the piston in a double acting cylinder except some volume is removed due to the space taken up by the piston rod.

Visualize the above diagram as the head end of a piston within a cylinder (blue dashed lines) and the left-hand side is top-dead center or at the head-end of the cylinder. The “Ps” line represents the suction or inlet pressure and the “Pd” line is the discharge pressure and the distance between is the difference or ratio.  Between “1” and “3” represents the length of the stroke and the area inside the heavy red lines represents the gas volume. Now that we are oriented we begin to move the piston within the cylinder.

Event 1 or Expansion -At point “1” the piston is at TDC (top dead center) or all the way to the top of its stroke. At this point there is still clearance between the piston and the end of the cylinder plus the valve ports. Within this clearance there is discharge-pressure as the discharge valves have just closed. We will assume there is no volume pocket or fixed clearance added to this cylinder. Added clearance will be explained later.

-The piston begins its suction stroke towards bottom-dead- center (BDC) or towards the bottom of its stroke. As the piston moves from point “1” to point “2” the discharge pressure left behind after the discharge valve closed, de-pressures and the gas volume consequently expands. When the piston reaches point “2” the gas has de-pressured enough to allow the suction valves to open and allow gas to enter the cylinder.

Event 2 or Suction -The piston continues past point #2 all the way to the bottom of its stroke (BDC) or to point #3. This distance between points #2 and #3 represents the volume of gas that entered the cylinder at suction pressure and at point #3 the suction valves close.

Event 3 or Compression –At point 3 the piston is at the bottom of its stroke – the suction valves have closed and begins its compression stroke towards the top of its stroke. The horizontal distance from point 3 to point 4 represents the distance the piston has to travel to compress the gas and raise the pressure to discharge pressure. When the piston reaches point 4 the pressure is slightly higher than the discharge pressure and the discharge valves open allowing gas to flow down the discharge line.

Event 4 or Discharge –The piston continues its travel from point 4 to point 5 which represents the volume of gas that was compressed and discharged from the cylinder at discharge pressure.  At point 5 the piston is at the top of its stroke and the cycle begins over again.


Adding Clearance to a Cylinder

            Cylinders may have adjustable volume pockets added to the head-end of cylinders or fixed clearance added to lower the cylinders capacity to accommodate changing inlet volumes or future changes that increase or lower the inlet gas or discharge volumes and pressures.  Another reason, if there is more than one stage, is optimizing the other stages.

-Giving a cylinder more clearance increases the distance between points 1 and 2, and the distance between points 3 and 4 -meaning the piston must travel a longer distance to de-pressure and expand the gas to the point where the suction valve will open and increases the distance it takes to compress the gas to the point where the discharge valves will open. Consequently this also shortened the distance between points 2, 3 and 4, 5 reducing the amount of gas allowed to enter and exit the cylinder – Thus reducing the efficiency and capacity of the cylinder.


Understanding what happens if suction or discharge pressures change.

            The distance between the “Ps” and “Pd” horizontal line on the PV diagram represents the difference or ratio between suction and discharge pressure.

-Visualize the suction pressure (Ps) to be 70 Kpa and the discharge (Ps) to be 210 Kpa resulting in a 3:1 ratio. —Now decrease the discharge pressure to 180 Kpa and leave the suction pressure at its original 70 Kpa, a ratio of 2.6:1.

-Results: The lower discharge pressure or ratio will reduce the distance or expansion time between points 1 and 2 increasing the amount of gas allowed to enter the cylinder by allowing the suction valve to open sooner. This change will also shorten the piston travel between 3 and 4 on the compression stroke before the discharge valve will open allowing more gas to exit the cylinder from points 4 to 5. This works the same way if we leave the discharge pressure and lower the suction pressure. Heat and horsepower are also reduced due to a shorter compression time. The amount of gas traveling through a cylinder determines how cool it will operate – the more the better. The opposite results happen as the ratio increases between suction and discharge and know that you must stay within the cylinders limits.

By John Goossens

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