Single Acting Compressor | Working, Applications and P-V diagram

The compressors are used to transfer the compressible fluids from one place to other. There ae multiple types of the compressors and single acting compressor is one of them. A single-acting air compressor is a famous type of positive displacement compressor from the category of reciprocating air compressors. In this article, I will explain the single-acting reciprocating air compressor and its P-V diagram. 

What is a Single Acting Compressor?

A single-acting compressor is a type of air compressor which uses only one end of the piston for the suction and compression purpose. In simple words, the first stroke of the piston sucks the air inside the compressor while the air compression occur in the second stroke.

In contrast, the piston’s other end is usually free or open. Due to this, we can’t perform any work by the other end of the piston. Therefore, only one side of the piston uses for sucking and compressing purposes. In simple wording, the piston upper part uses for air compression while the other side uses to open the crankcase. 

A single acting compressor uses compressed air to push the piston in one direction and the spring force to send the piston back to its primary position. It can work in the direction of pneumatic actuators. The cylinder of these reciprocating air compressors uses only one piston for supplying and discharging pressurized gas or air.

single acting compressor

A 3/2-way valve uses to control the single-acting reciprocating air compressor cylinder. The address 3/2 stands for 3 ports and 2 valve positions: One port is connected to the cylinder; the second post is connected with the compressed air source, and the third port is connected with the exhaust. The valve has two positions:

  • Inlet Position
  • Outlet Position

There are two valves; the inlet valve and the outlet valve. These both are check valves. Therefore, with a single-acting compressor, the whole compression process is carried out in only one step.

The opening and closing of a one-way valve depend on the pressure difference. When a mechanical valve is used for sucking and discharging, then the function of these valves’ controls through cams; when the piston is at TDC, the air weight in the compressor cylinder will be zero.

Read also: Different types of Compressors

Single Acting Compressor Working Principle

The single-acting air compressor working principle is very simple that is given below in detail.

single acting compressor working

This reciprocating compressor consists of an intake valve, an exhaust valve, a cylinder, valves, crankshaft and a piston.

As the piston moves in a downward direction, then the air pressure in the compressor cylinder drops lower than the pressure of the atmosphere. This pressure variation opens the inlet valve and draws air into the cylinder until the piston is completed its downward stroke.

As the piston moves up, the pressure builds up, and ultimately it touches the exhaust pressure. At this point, the outlet valve is opened for discharging the air. Then the air is sent to the storage tank.

At the end of the delivery stroke, a certain amount of high-pressure air remains in the clearance area.

When the piston stops the suction stroke, then the air in the clearance area increases until the air pressure drops below the pressure of the atmosphere. At this moment, the fresh air enters in the cylinder, the intake valve opens, and this cycle repeats.

Since the single-acting air compressor only has one valve at the top of the cylinder. And there is only one compression cycle for each revolution of the crankshaft.

Read Also: Working of Double Acting Compressor

Single Acting Air Compressor P-V diagram

The explanation of the P-V diagram is given below. The below figure represents a single-acting air compressor without clearance volume.

pv diagram of single acting reciprocating air compressor
                              P-V diagram

1) Suction Stroke (d-a):

First of all, the air is sucked into the compressor cylinder at constant pressure Pa and the volume of the air increases during this process. The line d-a represents this process in the above-given figure.

2) Compression Stroke (a-b):

After completing the suction stroke, the air compression starts. During this process, the air pressure increases from Pa to Pb, and the volume decreases from Va to Vb. At this point “b”, the pressure Pb is marginally more than delivery pressure. The line a-b represents this process.

3) Delivery stroke (b-c):

At point “b,” the compression stroke completes, and then the delivery stroke starts. During this process, the air pressure remains constant (Pb) while the volume increases. So, at this stage, the outlet valve opens and discharges the compressed air. The line b-c of the above-given diagram represents this process.

4) Repeating Process (c-d):

After completing the delivery stroke, the piston comes again for the suction stroke, again sucks air, and at this stage, the cylinder pressure will be Pa. This process repeats according to our requirements. And also, the work done during this process represents through a-b-c-d.

For better understanding of the working of single acting compressor, watch the following video:

Read also: Types of Reciprocating Compressors

Advantages and Disadvantages of Single Acting Reciprocating Compressor

Advantages of Single-Acting Compressors

There are multiple advantages to this type of reciprocating compressor that is given below.

  1. These compressors are less costly as compared to other types of reciprocating air compressors.
  2.  A single-acting reciprocating air compressor is less tubing.
  3. These are very easy to maintain.
  4. It uses less compressed air.
  5. These reciprocating compressors are best for cycles having medium duty.

Disadvantages of Single-Acting Compressors

  1. The major disadvantage of the single-acting air compressor is that it has a piston that works in only one direction at a time.
  2. This piston air compressor forces the air into the cylinder in only one direction.
  3. The spring takes up space and bounds the cylinder’s working stroke.
  4. The spring force of these compressors reduces the aerodynamics and limits the cylinder resulting force.
  5. These are less efficient than centrifugal compressors.

Applications of Single Acting Compressors

  1. These compressors use in internal combustion engines (diesel engines and petrol engines).
  2. They also use in external combustion engines.
  3. Single-acting reciprocating compressors also use in hydraulic rams and pumps.

In this article, I try my best to explain the single-acting air compressor working principle. I also profoundly explain the P-V diagram of it. If you have any questions, please feel free and let me know your problem in the comment section. I will try my best to give you a quick and ethnic response.

See More
  1. What is a Single-stage Compressor?
  2. How does a Reciprocating Compressor work?
  3. Centrifugal Compressor
  4. How does a Double Acting Compressor Work?

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