The importance of choosing the type of compressor in a compression chiller

In the air conditioning industry, any refrigeration system that produces chilled water is called a chiller. Chillers work based on the principles of condensation or absorption refrigeration. Compression chillers use electromechanical energy to start the cooling cycle, while absorption chillers are driven by heat for the cooling process in the refrigeration cycle. The compression chiller system, which is widely used in commercial and even residential buildings today, consists of 4 main components:

1. Compressor
2. Evaporator
3. Condenser
4. Expansion Valve
5. The compressor is the most important equipment in any compression chiller, and the classification of compression chillers is generally based on the type of compressor used in their structure:
6. Chiller Reciprocating
7. Chiller Screw (ChillerScrew)
8. Chiller Centrifugal (ChillerCentrifugal)
9. Chiller Scroll (ChillerScroll)

On the other hand, compression chillers are the biggest energy consumers in buildings. Therefore, it is important to get information about different types of chillers, their features, advantages, and limitations to make the right technical, engineering, and economic decisions for choosing, purchasing, and implementing chillers.

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Types of compressors in compression chillers

Most refrigeration systems, from home air coolers to large commercial and industrial chillers, use the refrigeration process known as the compression cycle. At the heart of every compression refrigeration cycle is the compressor, and its two tasks are a) to condense the gaseous refrigerant in the system to condense it and turn it back into a liquid and absorb heat from the air or water that is cooled and b) to pump the liquid refrigerant in the system It is cold.

Refrigeration and cooling systems do not have the same capacity, that's why chillers are classified according to the type of compressor:

1. Positive-Displacement Compressor
2. Dynamic Compressor (Dynamic Compressor)

Each of these compressors uses a special and sometimes innovative method to increase the vapor pressure of the refrigerant.

1. Positive displacement compressors mechanically condense the evaporated refrigerant to a higher pressure and smaller volume and are divided into reciprocating, rotary, and scroll types. These compressors transfer a constant volume of gas at a constant speed.
2. Dynamic compressors increase the pressure of the vaporized refrigerant through the kinetic energy exerted by a rotating impeller. Centrifugal compressor, which has attracted the attention of experts today in high capacities, is a dynamic compressor and is not a fixed displacement type.

Types of compression chillers based on the type of compressor

As mentioned earlier, the classification of compression chillers is generally based on the type of compressor used in the chiller structure. To get familiar with compression chillers, in the following, different types of chillers, advantages, disadvantages, limitations, and applications of each are described in detail.

Reciprocating Chillers

Reciprocating compression chillers are equipped with reciprocating compressors (cylinder and piston) in their structure. Reciprocating compressors are positive displacement machines that use the reciprocating movement of a piston inside a cylinder to condense refrigerant gas. As the piston moves down, a vacuum is created inside the cylinder. Because the pressure above the inlet valve is higher than the pressure below it, the inlet valve is opened and refrigerant is sucked into the cylinder. After the piston reaches its bottom point, it starts to rise. At this time, the inlet valve is closed and the refrigerant is trapped inside the cylinder. As the piston continues to move upwards, the refrigerant condenses and its pressure increases. At a certain point, the pressure of the refrigerant leads to the opening of the discharge valve and transfers the flow of compressed refrigerant to the outside of the cylinder. When the piston reaches its top point, it starts to go down and the cycle repeats.

These compressors are available in three structures: a) Hermetic, b) Semi-Hermetic and c) Open.

1. In a closed compressor, the motor and the compressor are placed in a single body, which is sealed. Since the parts are not accessible for repair in these compressors, the entire compressor must be replaced in case of possible failure. Chillers equipped with sealed closed compressors are common for low capacities.
2. In a semi-enclosed compressor, the motor is also part of the unit, but the casing is not sealed. Semi-enclosed compressors have many advantages over closed compressors because they can be repaired many times and have a longer life.
3. In an open compressor, the motor and the compressor are separated by a flexible coupling. These types of chillers use older technology and are not common today.

The most important features of the reciprocating chiller are its low cost and low efficiency when used in low capacities. Also, in these chillers, reciprocating compressors can be installed in multiples to provide more building loads. Other advantages of these systems are ease to control and adjustable speed. In general, chillers with reciprocating compressors have a strong structure and are suitable for operating in adverse weather conditions such as in the southern regions of Iran. Reciprocating chillers are available in two types air-cooled and water-cooled in terms of heat removal structure, and these chillers can be supplied in capacities of 5 to 150 tons of refrigeration.

Figure 4- Reciprocating air chiller

Figure 5- Reciprocating water chiller

The main drawback of reciprocating chillers is that they require a lot of repairs and maintenance compared to other new chillers. Reciprocating chillers have more moving parts than centrifugal or rotary chillers, resulting in an increased need for wear-related maintenance activities. Reciprocating compressor chillers also produce a lot of noise and vibration. Therefore, special considerations should be made to isolate the high noise of the chiller from the unit to prevent the transmission of machine vibrations and sounds to the building.

Finally, it can be said that reciprocating chillers are not suitable for applications with a cooling load of more than 200 tons. By increasing the capacity of the units, the space limitations and initial purchase costs of these chillers exceed other types of chillers. In addition, if you choose the above chillers, the energy consumption of larger units will be higher compared to other chillers.

Screw compression chillers (Screw Chillers)

Screw compression chillers are a type of chillers that have a screw or screw compressor in their structure. Screw compressors are positive displacement machines that use spiral rotors to condense refrigerant gas. As the rotors rotate, they are engaged and the intermediate spaces at the ends of the rotors are alternately opened and closed. When the intermediate space created at the end of the inlet is opened, refrigerant is sucked into it. As the rotors continue to rotate, the refrigerant is trapped in the middle space, and forces are applied to it along the length of the rotors. Finally, the volume of the middle space is reduced and as a result, the refrigerant is condensed. The condensed refrigerant also escapes when the interspace reaches the other end. In terms of structure, there are two types of screw compressors:

1) Single-Screw Compressor

2) Twin-Screw Compressor

The single screw compressor uses a main screw rotor that is connected to two gate rotors with matching teeth. In this compressor, the main screw is driven by the prime mover, which is mainly an electric motor.
A twin-screw compressor consists of precisely tuned rotors (one male and one female) that are in close contact with each other as they rotate inside a casing. The operation of this compressor is that one rotor is moved and the other rotors rotate in the opposite direction. A twin-screw compressor has better controllability and allows changes in suction pressures without greatly affecting operating efficiency.
Screw compression chillers are available in both air and water chiller designs.

Figure 8- Screw air chiller

Figure 9- Screw blue chiller

With a relatively high compression ratio and few moving parts, compact screw chillers are smaller and lighter than reciprocating and centrifugal chillers of similar cooling capacity. Also, the operation of these chillers is quieter and without vibration, so screw chillers are known for their strong structure, simplicity, and reliability. Compression chillers equipped with screw compressors are designed for long periods of continuous operation with little need for repairs and maintenance. The compressors in these chillers can overcome the high lift force when the speed decreases and lead to energy savings without the possibility of the compressor surging during load reduction.
The main disadvantage of compression chillers with screw compressors is the high initial purchase cost. For low cooling loads, reciprocating chillers are cheaper to purchase and install; Centrifugal chillers are also cheaper for large loads.
The range of the applied capacity of screw chillers with one compressor is usually from 80 to 250 tons of refrigeration, and if several compressors are used in the chiller structure, this capacity increases to 750 tons of refrigeration.

3. Centrifugal compression chillers (Centrifugal Chillers)

Centrifugal compression chillers are equipped with centrifugal compressors in their structure. A Centrifugal compressor is a dynamic machine that uses the rotary movement of an impeller wheel to apply centrifugal force to the refrigerant inside a circular chamber (volute). The refrigerant is sucked into the impeller by a large circular inlet and the refrigerant flows between the impellers. The force of propellers pushes the refrigerant to the outside and applies centrifugal force to it. Refrigerant pressure increases due to forced movement and hitting the walls of the circular chamber. Centrifugal compressors are suitable for condensing large volumes of refrigerant to relatively low pressures. The condensing force created by the impeller is small, so chillers equipped with centrifugal compressors usually employ more than one impeller in series. Today, centrifugal compressors, which are the latest model of compression chiller compressors, are popular due to their simple design and few moving parts.

Centrifugal chillers are positive pressure and negative pressure machines based on the evaporator pressure conditions and the type of refrigerant used. A chiller that uses R-22 and R-134A refrigerants is a positive pressure machine while a chiller that uses R-123 refrigerants is a negative pressure machine.
Like reciprocating chillers, centrifugal chillers are available in Hermetic Sealed and Open structures. Despite the lower operating efficiency, the sealed-type centrifugal compressor is used on a broader scale.

Figure 11- Centrifugal water chiller

Advantages of centrifugal chillers

The most important advantage of the centrifugal compression chiller is the high-performance efficiency in full load mode. Unlike reciprocating and rotary screw compressors, the centrifugal compressor is not a fixed displacement machine, so the centrifugal chiller covers a wide range of capacities at a limited range of pressure ratios. These chillers are suitable for condensing large volumes of refrigerant to relatively low pressures. By varying design factors such as the number of stages, compressor speed, impeller diameter, and refrigerant type, centrifugal compressors can be used in chillers that have a wide range of design temperatures for both the refrigerant and the refrigerant.
Centrifugal compressors have conquered the market of chillers of 200 tons of refrigeration and above due to their very high capacity of refrigerant vapor flow. Because they have the cheapest and most efficient design in that capacity range. Centrifugal chillers are often started by electric motors, but they can be driven by steam turbines or gas engines.

Disadvantages of centrifugal chillers

The most serious problem with centrifugal chillers is their performance in partial load mode. When the building load decreases, the chiller responds by partially closing the inlet valves to limit refrigerant flow. While this control method is effective for capacities up to about 20% of the nominal output of the chiller, it also leads to a decrease in performance efficiency. Since chillers operate at or near full load less than 10% of the time, the performance characteristics of part-load operation significantly influence annual energy consumption.
On the other hand, the operation of centrifugal chillers in low cooling loads is difficult because when the load drops to 25% of the nominal output of the chiller, the chiller is at risk of surging. To reduce the possibility of surging that causes serious damage to the chiller, manufacturers add special controls, but most of these controls are associated with a reduction in the partial load efficiency of the unit.
Also, the higher initial purchase price of centrifugal chillers under 200 tons of refrigeration, compared to screw and reciprocating chillers, is one of their other limitations.

4) Scroll Chillers

Scroll compression chillers are chillers whose compressors are of the scroll or spiral type. A scroll compressor is a positive displacement machine where the refrigerant is compressed by two spiral disks. The upper disk is stationary while the lower disk moves in an orbital motion. The orbital motion of the lower disc creates sealed spaces of variable volume within the fixed disc. Refrigerant gas enters through the inlet located on the surface of the snail. Part of the refrigerant is trapped in one of the sealed spaces. With the movement of the disk, the closed space containing the refrigerant moves towards the center of the disk, and its volume decreases, and as a result of this decrease in volume, the refrigerant condenses. Finally, the condensed refrigerant is discharged through a port located in the center of the upper disk.

Scroll chillers have made a lot of progress in recent years so they are quickly taking the place of reciprocating chillers. These chillers have small dimensions, low noise and vibration, and good efficiency. Chillers equipped with scroll compressors are available in air-cooled (air) and water-cooled (water-cooled) structures, and their capacity reaches approximately 30 tons of refrigeration or less, which makes them a good choice for applications in small buildings and It has become special for residential buildings.
The biggest disadvantage of chillers equipped with scroll compressors is that these compressors cannot be repaired. Scroll compressors also have problems due to oil loss at low temperatures.

Figure 13- Scroll air chiller

Figure 14- Scroll water chiller 

Comparison of compression chillers in terms of cooling capacity

The size of the compressors used in compression chillers is expressed based on the horsepower of the input engine (HP), kilowatt power of the input engine (kW input), refrigeration capacity (kW cooling), a ton of refrigeration (TR), or (BTU/hr). . In fact, the volume and pressure in which the refrigerant can be pumped determine the cooling capacity of the compression chiller, its performance and the application areas in which the chiller can be used with that particular compressor.
Reciprocating chillers are made in capacities of 5 to 150 tons of refrigeration. The power consumption of reciprocating air chillers is about 1 to 1.3 kW per ton of refrigeration (kW/TR), while the power consumption of water chillers is about 0.7 to 1 kW per ton of refrigeration (kW/TR). (water chillers are more efficient than air chillers)
Screw air chillers are available in cooling capacities of about 70 to 500 tons of refrigeration and with a consumption power of 1.1 to 1.5 kW per ton of refrigeration. The cooling capacities of screw water chillers are between 70 and 750 tons of refrigeration and their consumption power is around 0.65 to 0.9 kW per ton of refrigeration.
Centrifugal chillers mainly have about 90 to 1000 tons of refrigeration capacities, most of which are in the capacity range of 150 to 300 tons of refrigeration. Centrifugal compressor chillers are the most efficient chillers in terms of energy consumption, and their consumption power is between 0.5 and 0.6 kW per ton of refrigeration!

Important recommendations

In cases where the peak load reaches 80 to 100 tons of refrigeration, chillers equipped with reciprocating compressors are a suitable option. For peak loads between 100 and 200 tons of refrigeration, two or more reciprocating chillers can be used.
For peak loads above 200 tons, chillers equipped with screw compressors start to become economically efficient. Screw chillers are suitable for applications of up to 750 tons of refrigeration. Above these capacities, centrifugal chillers are more economical if water is available for heat removal.
Chillers equipped with centrifugal compressors mainly cover capacities larger than 750 tons of refrigeration. Centrifuge machines have the highest peak load efficiency and work reliably for applications that require stable and uniform activity. These chillers are only recommended with the water-cooled condenser option.
It is reminded again that the efficiency of water chillers is higher than the efficiency of air chillers.

Comparison of condensing chillers in terms of investment costs (Capital Costs)

Investment costs are the lowest for reciprocating compression chillers and the highest for screw compression chillers.

• Reciprocating chillers are more common and generally less expensive than other chillers, especially for smaller capacities.
• Screw chillers are the most expensive option in terms of the initial cost.
• Centrifugal chillers are 10-15% cheaper than screw chillers in most sizes and under the same operating conditions. The initial cost of the centrifugal chiller is higher than the reciprocating chiller in capacities less than 200 tons of refrigeration, but it competes with it in higher capacities.

Comparison of compression chillers in terms of efficiency.

Chiller efficiency is expressed based on kilowatts per ton of refrigeration (kW/TR) or its coefficient of performance (COP), which defines the coefficient of performance as the ratio of output thermal energy in terms of BTU to input thermal energy in terms of BTU.

• Reciprocating air chillers need 1 to 1.3 kW per ton of cooling power depending on the capacity and ambient air temperature at peak load. Therefore, the peak load performance factor for these systems is in the range of 3.52 to 2.7.
• Conventional screw chillers with water-cooled condensers require about 0.6 to 0.8 kW/ton of refrigeration at peak load, resulting in a performance factor of 5 to 7. The efficiency of the screw compressor and reciprocating compressor is equal when they operate at the same compression ratio. Screw compressors can operate in 30 compression ratios, and this feature enables them to work more efficiently and better than reciprocating compressors.
• Centrifugal chillers are the most efficient option at peak load and consume the least amount of power. According to the standards related to operational conditions, the centrifugal chiller at full load and for capacities of more than 300 tons of refrigeration requires a power of 0.53 kW per ton of refrigeration, and for capacities of less than 300 tons of refrigeration, the required power is between 0.6 and 0.7 kW. It has refrigeration in it. The efficiency of these chillers has improved over the years as a result of improved impeller designs, better chiller structures, increased heat transfer levels, and increased emphasis on reducing chiller energy demands.

The approximate values of power consumption in each of the compared compression chillers are given in the table below.

Table 1- Approximate rate of power consumption in chillers in full load mode

Comparison of compression chillers in terms of performance in partial load mode (Part Load Performance)

In air cooling systems, peak load occurs during only a limited number of hours during the year. On an annual basis, the building load changes based on the time of day and as a result the present pattern of occupants, thermal energy absorbed from the sun during the day, etc., and also based on the time of year due to seasonal changes in solar radiation and temperature. Despite all these variables, any water compression chiller operates in off-design conditions most of the time. Various studies show that a chiller operates only 1% of the time at 100% capacity, 42% of the time at 75% capacity, 45% of the time at 50% capacity, and 12% of the time at 25% capacity. Therefore, the partial load performance of a system is the basic factor in choosing a chiller. The items related to partial load efficiency of different options are stated below:

• The screw chiller has an unlimited efficiency reduction from 100% load to 0% load and has good partial load efficiency overall.
• The centrifugal chiller reduces the capacity by reducing the stages. When the building load decreases, the chiller reacts by partially closing the inlet valves and restricting the refrigerant flow. While this control method is effective for capacities up to 20% of the chiller's nominal output and leads to a decrease in chiller performance. As an example, a chiller with a power consumption of 0.6 kW per ton of refrigeration in full load mode may require 0.9 kW per ton of refrigeration power when the load is reduced.
• Step capacity control of reciprocating compressors at minimum load is more efficient than screw compressor operation. It is easy to install multiple reciprocating compressors to adapt the capacity of the chiller to the load of the building, and it allows the unit manager to determine how the system works in partial load conditions, and this leads to an increase in performance efficiency.
• One of the best choices to have high efficiency in partial loads of medium and small buildings is to use a chiller with the right number of scroll compressors.
  

  You can also read the article MOTOR VALVE AND ITS APPLICATION IN HEATING AND COOLING SYSTEMS for more information.

Comparison of compression chillers in terms of useful life (Economic Life)

Comparison of compression chillers in terms of useful life (Economic Life)
The useful life of the chiller is a period in which the chiller can be used by the consumer, and after its completion, it is depreciated and it is expected that it can no longer be used physically. When the operation and maintenance of a component of the equipment If it costs more than replacing it, its useful life is over.
The useful life of chillers equipped with reciprocating compressors is 15 years, while chillers with rotary and centrifugal compressors are expected to have a useful life of 25 years.

Final speech

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