1.  Introduction:

The compressor is a device that is used to increase the pressure of compressible fluid from low pressure to high pressure by using some external energy. In the refrigeration systems, the compressors used to compress the vapor refrigerant, hence raising its pressure from evaporator pressure to condenser pressure. The pressure at both suction and discharge are related, corresponding with the type of compressor.

The compressors can be classified into two types based on the working principle:

1-positive displacement type

2-dynamic type

In positive displacement compressors, a certain volume of compressible fluid is drawn and trapped by closing both suction and discharge valves. In dynamic compressors, the kinetic energy is delivered to the compressible fluid by gas by velocity or the centrifugal force and then convert to pressure energy.

Depending on the operating conditions, site requirements, and optimal uses, there are several differences to consider when deciding between a centrifugal and reciprocating compressor for an application.

 

Operators have a lot of options for compressors, when determining which solution works best, there are four main components to be especially aware of:

·        Overall cost 

·        Flow rate 

·        Efficiency 

·        Multi-staging capability 

 

Reciprocating Compressor

A reciprocating compressor is a positive displacement compressor, it is like an IC engine (consisting of cylinders, pistons, crankshaft, connecting rod, and inlet and outlet valve) in which the vapor refrigerant is compressed by back-and-forth (reciprocating) motion of the piston. This kind of compressors is used for refrigerant which has large differential pressure and low specific volume such as (NH3, R-22, R12, R-40).

In general, there are two types of reciprocating compressors of which are:

1.     Single acting vertical compressors, usually the cylinders are arranged in V or W form or arranged vertically.

 

2.     Double acting horizontal compressors, usually the cylinders are arranged horizontally. The available sizes of reciprocating compressors are almost 100 watts which are used for small refrigerators for domestic application up to 150 kW for large capacity installation

Fig.1.Cut away view of multi-cylinder compress

A reciprocating compressor utilizes pistons for compressing air and has a design that resembles that of an internal combustion engine. These pistons sit inside cylinders and are central to the primary function of the compressor. 

For compression, the pistons draw back as gas gets injected from an intake valve in the compressor. After the gas is injected into the cylinders, it’s compressed by the reciprocating motion of the pistons.

The advantages of a reciprocating compressor over a centrifugal machine are:

1. Greater flexibility in capacity and pressure range,
2. Higher compressor efficiency and lower power cost,
3. Capability of delivering higher pressures,
4. Capability of handling smaller volumes,
5. Less sensitive to changes in gas composition and density.

           Fig.2.Estimated efficiency as a function of compression ratio reciprocating compressors

 

Working Principle of Reciprocating Compressors

 

The piston at the top is called top dead centre in this position both valves (inlet and outlet) are closed

fig.(A) when the piston move down the refrigerant trapped in clearance volume expands

fig(B), hence the volume above the piston increase, consequently the pressure decrease when the pressure becomes lower then suction pressure the inlet valve gets opened

fig(C) and the refrigerant flow through it into the cylinder until the piston reaches bottom dead centre

fig(D), when the piston starts moving upward the volume above piston gets decreased and the pressure of refrigerant increases, once the refrigerant pressure becomes greater than the pressure applied to the outlet, consequently, the valve gets opened and the vapor refrigerant is discharged into the condenser and the cycle is repeated. The process is represented on a pressure-volume diagram. The area of the diagram represents the work required for the compressor to compress and discharge the compressible fluid.

 

Fig.3.Working Principle of Reciprocating Compressors

 

 

 

 

Fig.4.Pressure–volume diagram

 

Work in Reciprocating Compressors

 

The work done for compression s given by the cyclic integral of pdV (contour integration of pdV). Hence

            Fig.5.Cylinder and piston mechanism and p-V diagram of a reciprocating compressor

 

 

Pros and Cons of Reciprocating Compressors Ø

 

Pros:

1.     Discharges high-pressure gas.

2.     Have High efficiency (high speed reciprocating 72 – 85%, low speed reciprocating 75 – 90%)

3.     Have low power consumption

4.     Early design and production, mature manufacturing technology. Ø

 

Cons:

1.     The size of the compressor is large for a given capacity.

2.     High vibration and noise due to many moving parts that the compressor consists of (piston, connecting rod, crankshaft, … etc.)

3.     Part of the work input is lost due to friction between the piston and cylinder.

4.     The speed is limited.

 

Applications of Reciprocating Compressors

1.     Mainly used in the refrigeration and air conditioning plants

2.     Natural gas processing plants, oil refineries,

3.     Used in blowing of the plastic bottle

Centrifugal Compressor

 

The centrifugal compressors a dynamic –type compressors and are second only to reciprocating compressors in the numbers of machines in service. the centrifugal compressor acts on increasing the pressure of the low-pressure compressible fluid (refrigerant; in refrigeration and air conditioning applications) to high pressure by centrifugal force. usually used for refrigerants that require low condensing pressure such as (R-11, R-113, R-717) and R-12 used for low temperature and large capacity applications, also used for the temperature range of (-90 to 10 C).

 

 

 

Centrifugal Compressors have two main types of which are:

1. Single-stage centrifugal compressors.

2. Multi-stage centrifugal compressors.

 

Single-stage centrifugal compressor mainly consists of the following parts:

1. An inlet casing: to accelerate the compressible fluid (vapor refrigerant) to the inlet of the impeller.

2. An impeller: to transfer energy to the compressible fluid in form of kinetic energy and (enthalpy (rise in pressure).

3. A diffuser: to convert KE at the impeller outlet into enthalpy (causing pressure rise).

4. A volute casing: to collect the compressible fluid and converting the remaining KE into enthalpy.

 

 

Fig.6.The main parts of single-stage centrifugal compressor

Fig.7.Multi-stage centrifugal compressor

 

Fig.8.Estimated efficiency as a function of compression ratio centrifugal compressors

 

A centrifugal compressor is a turbocompressor or dynamic compressor which has a radial design. This configuration works at constant pressure, which means that the performance is primarily affected by inlet temperatures and other external factors. 

It works by drawing air into the core of a rotating impeller with radial blades. The air is pushed into the centre by centrifugal force, thus resulting in rising pressures and kinetic energy. This energy is then converted into pressure by passing through a diffuser and volute. 

A centrifugal compressor is a popular option for:

·        Oil and gas

·        Process industries 

·        Wastewater treatment plants 

High and low-pressure variants are used depending on specific applications and industries. 

 

The advantages of a centrifugal compressor over a reciprocating machine are:

1. Lower installed first cost where pressure and volume conditions are favorable,
2. Lower maintenance expense,
3. Greater continuity of service and dependability,
4. Less operating attention,
5. Greater volume capacity per unit of plot area,
6. Adaptability to high-speed low-maintenance-cost drivers.

 

Working Principle of Centrifugal Compressors

Single-stage centrifugal compressor is the simplest form. The impeller draws the low pressure compressible fluid (vapor refrigerant) from the evaporator. Once the impeller rotates, the vapor refrigerant is pushed from the center to the outer edge of the impeller (periphery) through centrifugal force. The high rotating speed of the impeller keeps the fluid at high speed at the vane tips of the impeller. When high-velocity fluid passes through a diffuser the KE energy of the fluid is converted to enthalpy (rise in pressure) then the volute casing collects the fluid from the diffuser and converts the remaining KE of fluid to pressure energy before leaving the compressor.

 

Fig.9.Working Principle of Centrifugal Compressors

 

Work in Centrifugal Compressors

Pros and Cons of Centrifugal Compressors Ø

Pros:

1. The operation of centrifugal compressors is quite .
2. The efficiency of centrifugal compressors is high (72 – 85%).
3. They have fewer rubbing parts; hence, longer working life as compared to the reciprocating compressors.
4. Wide range of rotational speed (3000 r.p.m and above)
5.  Higher flow rate as compared to positive displacement compressors. Ø

Cons:

1. The main disadvantage of centrifugal compressors is surging (when the refrigeration load gets decreased, the refrigerant flows back from the compressor to the evaporator, this is so-called “surging”).
2. High compression ratio is not achievable, since the increase in pressure per stage is lower than reciprocation compressors.

Applications of Centrifugal Compressors

1.     Used in diesel engine turbocharger and supercharger.

2.     Refrigeration, air conditioning, and HVAC.

3.     In gas turbines used to provide compression.

4.     Used to supply compressed air for all types of pneumatic tools.

Comparison of Performance of Reciprocating and Centrifugal Compressors

𝐭𝐤 :𝐂𝐨𝐧𝐝𝐞𝐧𝐬𝐞𝐫 𝐭𝐞𝐦𝐩𝐞𝐫𝐬𝐭𝐮𝐫𝐞 (C) , 𝐭𝟎 :𝐄𝐯𝐚𝐩𝐨𝐫𝐚𝐭𝐨𝐫 𝐭𝐞𝐦𝐩𝐞𝐫𝐚𝐭𝐮𝐫𝐞 (C), 𝐍: 𝐒𝐩𝐞𝐞𝐝 𝐨𝐟 𝐫𝐨𝐭𝐚𝐭𝐢𝐨𝐧 (r.p.m), 𝐁𝐇𝐏: 𝐃𝐫𝐢𝐯𝐢𝐧𝐠 𝐩𝐨𝐰𝐞𝐫 𝐫𝐞𝐪𝐮𝐢𝐫𝐞𝐝 (), 𝐐′𝟎 :𝐑𝐞𝐟𝐫𝐢𝐠𝐞𝐫𝐚𝐭𝐢𝐨𝐧 𝐞𝐟𝐟𝐞𝐜𝐭 (TR)

 

The evaporator temperature for centrifugal compressors changes from almost 2 to 8 C for the load variation of 100 to 250 TR, while for reciprocation compressors the temperature in the evaporator varies from almost -12 to 6 C for the same load variation.

Fig.10.Load variation vs. evaporator temperature

For centrifugal compressors, once the condensing temperature increases, the mass flow rate of refrigerant through the compressor gets decreased (ultimately, the refrigeration effect decreases); hence, there will not be overloading of the motor as the condensing temperature increase. On the other hand, there is a slight decrease in refrigeration capacity; hence, there will be overloading of the motor with a high condensing temperature.

Fig.11.Load variation vs. Condensing temperature

For centrifugal compressors, the power required to drive the compressor decreases with increasing the condensing temperature, unlike reciprocating compressors in which the power required gets increased as the condensing temperature increase.

Fig.12.B.P. variation vs. Condensing temperature

Some distinguishing points about centrifugal and reciprocating compressor

Reciprocating—Can be applied with suction pressures at atmospheric or even a slight vacuum. In vacuum applications, precautions must be taken to prevent atmospheric air from leaking into the cylinder through the piston rod packing.

Centrifugal—Inlet pressures to atmospheric or below. For subatmospheric inlet conditions, special seal and buffering designs are employed to keep atmospheric air from being drawn into the compressor.

Maximum Flow

Reciprocating—Reciprocating compressors are positive displacement type compressors. Capacity is limited by cylinder size, the number of throws available, and the available driver speeds. A “throw” is a location on the crankcase where a compressor cylinder can be attached.

Centrifugal—Centrifugal compressors can be sized for an inlet flow of 400,000 acfm (680,000 m3/hr) in a single body. The maximum flow through a centrifugal compressor is limited by the choke point, which is the point at which the flow through some part of the compressor nears a velocity of Mach 1.

Minimum Flow

Reciprocating—Similar to the maximum flow, the minimum flow in a reciprocating compressor is limited by cylinder size, stroke, and speed. Very small reciprocating compressors are available.

Centrifugal—Centrifugal compressors can be sized for flow as low as a few hundred acfm. Unlike a reciprocating compressor where minimum flow is solely a function of compressor geometry and speed, the minimum flow for a centrifugal compressor is limited by an aerodynamic condition known as surge, which is a function of compressor geometry, speed, aerodynamic gas conditions, and system resistance.

Flow Range

Reciprocating—Reciprocating compressors have the ability to change flow (throughput) through speed control, the addition of fixed clearance to a cylinder (fixed or variable volume clearance pockets), cylinder end deactivation, and system recycle. Typical flow range might be 100 percent down to as low as 20 percent, and even lower.

Compressed Gas Molecular Weight

Reciprocating—A reciprocating compressor has no limit with regard to molecular weight. Very light and very heavy gases are compressed equally well. Over the range of molecular weight different application configurations may be required. For example, very low molecular weight gases may present some seal challenges and very high molecular weight gases pose issues with efficiency. But nonetheless, the recip handles the whole range quite well.

Centrifugal—Compression ratio is highly dependent on molecular weight. Head is developed by increasing gas velocity to create kinetic energy and then converting the kinetic energy to pressure in the diffuser. The amount of kinetic energy is a function of gas velocity and mass or molecular weight. Centrifugal compressors are used for a broad range of molecular weight including low molecular weight applications such as hydrogen recycle and high molecular weight applications using refrigerant gases with molecular weights over 100.

 

Compression Ratio

Reciprocating—The maximum compression ratio that a reciprocating can handle in one stage is limited mostly by compressed gas discharge temperature. The piston rod load generated by the compression ratio may also be a limit. Typical compression ratios are 1.2 to 4.0.

Centrifugal—Compression ratio is a function of gas molecular weight, compressibility, stage geometry ,compressor speed, and the number of compressor stages. For a specific gas, the limits to compression ratio are the mechanical and rotor dynamic limitations on speed and the number of stages that can be accommodated in a single body. Discharge temperatures resulting from high compression ratios can usually be controlled by intercooling.

Efficiency

Reciprocating—Reciprocating compressors have a very characteristic adiabatic efficiency curve .As compression ratio drops, adiabatic efficiency drops. Efficiency changes with molecular weight. Efficiency will also vary with several other factors, most significantly the compressor cylinder’s ratio of valve flow area to main bore diameter and piston speed.

Reciprocating Compressor Efficiency.

Centrifugal—Polytropic efficiency is typically used for centrifugal compressors rather than adiabatic. Adiabatic is commonly used for air compressors. Typical polytropic efficiencies range from 70 percent to 85 percent.

Efficiencies approaching 90 percent are possible. In a centrifugal compressor, efficiency is primarily affected by the internal leakage and mechanical losses.

Cost: Capital and Operating

Reciprocating—Generally a reciprocating compressor will have a lower capital cost since centrifugal use complex geometry parts but recips have a higher operating cost compared to a centrifugal. A centrifugal compressor has fewer wearing parts, resulting in lower operating costs in terms of replacement parts, repairs, and downtime.

Conclusion and Recommendations

Compressor, this device provides a very important service of which is transferring potential energy stored to the pressurized compressible fluid to perform work. There is a wide range of compressors’ types used for different application, each type has its features and characteristics. Since the report represents the centrifugal and reciprocating compressors, so as a professional, one cannot be assured that a centrifugal compressor is better than a reciprocating one or vice versa, where the decisive factor remains the application or case for which the compressor will be used. But in general centrifugal compressors in refrigeration and air conditioning applications represents better characteristics than reciprocating compressors such as low vibration, higher discharged flow rate than reciprocating, quietness during operation, changing of the temperature inside evaporator which is lower for the same load variation as compared to reciprocating compressors, as well as non overloading characteristic. In conclusion, the centrifugal compressors will be a preferable choice, except when a very high compression ratio is required.

References

1.     Dilyar O. Junedi “Reciprocating and Centrifugal Compressors”, Ministry of high education and scientific researches Erbil Polytechnic University Erbil Technical Engineering College Mechanical & Energy Engineering Department

2.     https://kbdelta.com/blog/centrifugal-vs-reciprocating-compressor.html