What Is a Chiller and How Does It Work? Everything You Need To Know About Chillers. God देव and सत्य Satya on every grain of earth since the beginning of time before 2035

Chiller Overview

A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream (such as air or process water). As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function. Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional facilities. Water or liquid chillers can be liquid-cooled, air-cooled, or evaporatively cooled. Water or liquid-cooled systems can provide efficiency and environmental impact advantages over air-cooled systems. Chilled water systems are used in medium and large-sized buildings. Chiller plants act as a centralized cooling system that provides cooling for an entire building or even multiple buildings. 

A typical water-cooled chilled water system consists of 4 major components as below:

• Chiller
• Cooling Tower
• Chilled Water Pump
• Condenser Water Pump

How Does A Chiller work?

In most cooling applications, a pumping system circulates cool water or a water/glycol solution from the chiller to the field units. This cool fluid removes heat from the field units and the warm fluid returns to the chiller. Chillers contain a chemical compound, called a refrigerant. There are many types of refrigerant and applications depending on the temperatures required but they all work on the basic principle of compression and phase-change of the refrigerant from a liquid to a gas and back to a liquid. This process of heating and cooling the refrigerant and changing it from a gas to a liquid and back again is the refrigeration cycle.

The refrigeration cycle starts with a low-pressure liquid/gas mix entering the evaporator. In the evaporator, heat from the process water or water/glycol solution boils the refrigerant, which changes it from a low-pressure liquid to a low-pressure gas. The low-pressure gas enters the compressor where it is compressed to high-pressure gas. The high-pressure gas enters the condenser where ambient air or condenser water removes heat to cool it to a high-pressure liquid. The high-pressure liquid travels to the expansion valve, which controls how much liquid refrigerant enters the evaporator, thereby beginning the refrigeration cycle again.

• Compressor : The compressor takes low pressure and low temperature refrigerant and compresses it until it becomes a high pressure and temperature gas. Three of the types of compressors are centrifugal, turbocor, and screw. The compressor is the the prime mover, it creates a pressure difference to move the refrigerant around the system. There are various designs of refrigerant compressors, the most common being the centrifugal, screw, scroll and reciprocating type compressors. Each type has it’s own pro’s and con’s. It is always located between the evaporator and the condenser. It’s usually partly insulated and will have an electrical motor attached as the driving force, this will be either mounted internally or externally. Compressors can be extremely noisy, usually a constant deep droning sound with an overlaying high pitch, hearing protection should be worn when in close proximity to the chiller.

• Condenser : The compressed gas flows through coils in the condenser, where air or water moves over the coils to remove heat from the refrigerant. Once the refrigerant loses its heat, it condenses into a liquid. The condenser is located after the compressor and before the expansion valve. The purpose of the condenser is to remove heat from the refrigerant which was picked up in the evaporator. There are two main types of condensers, Air cooled and Water cooled. Water cooled condensers will repetitively cycle “Condenser water” between the cooling tower and the condenser, the hot refrigerant which enters the condenser from the compressor, will transfer its heat into this water which is transported up to the cooling tower and rejected from the building. The refrigerant and the water do not mix they are kept separated by a pipe wall, the water flows inside the pipe and the refrigerant flows on the outside. Condensers on air cooled chillers work slightly differently, they do not use a cooling tower but instead blow air across the exposed condenser pipes with the refrigerant flowing this time on the inside of the tube.

• Evaporator: In the evaporator, the refrigerant returns to being a gas, becomes very cold, and absorbs heat. It is in the evaporator where the refrigerant and fluid interact, where heat is removed from the fluid to be transferred to the refrigerant. Three common types of evaporators are copper coil, shell and tube, and plate. The evaporator is located between the expansion valve and the compressor, its purpose is to collect the unwanted heat from the building and move this into the refrigerant so that it can be sent to the cooling tower and rejected. The water cools as the heat is extracted by the refrigerant, this “chilled water” is then pumped around the building to provide air conditioning, This “Chilled water” then returns to the evaporator bringing with it any unwanted heat from the building.
• Expansion Valve : The expansion valve, which may also be known as a thermostatic or electronic expansion valve, controls the amount of refrigerant that passes between the condenser and evaporator and changes the flow based on the cooling load. The expansion valve is located between the condenser and the evaporator. It’s purpose is to expand the refrigerant reducing its pressure and increase it’s volume which will allow it to pick up the unwanted heat in the evaporator. There are many different types of expansion valve, the most common at the thermal expansion vale, the pilot operated thermal expansion valve, the electronic expansion valve and the fixed orifice expansion valve.
• Pump: The pumping system circulates cool water, or a water/glycol solution, from the chiller to the process to be cooled
• Filter: The filter is designed to capture contaminants, dirt, and particles that may enter the chiller fluid. They are also part of the air intake system.
• Power Unit and Controls – The power unit is either mounted directly to the chiller or it can be separated and mounted to the wall of the plant room with power cables running between them. The purpose of the power unit is to control the flow of electrical power to the chiller. These usually contain a starter, circuit breakers, speed controller and power monitoring equipment. The controls unit is typically mounted on the chiller. It’s purpose is to monitor the various aspects of the chillers performance and control these by making adjustments. The controls unit will generate alarms for the engineering teams and safely shut the system down to prevent damage to the unit. BMS connections are also usually present to allow remote control and monitoring.
• Water boxes: : Water boxes are mounted to the evaporators and also the condensers of water cooled chillers. The purpose of the water box is to direct flow as well as as to segregate the entrance and exit. Depending on the number of passes in the evaporator and condenser, water boxes may have 1-2 flanged entrance or exit holes or they can be completely capped and just redirect flow back into the next pass.

Types of Chillers

• Water Cooled Chillers: Water cooled chillers are normally combined with a cooling tower and use a condenser water treatment system to remove mineral deposits. The cooling tower sends water to the chiller to be cooled. Water-cooled chillers are almost always located inside of a building. They work almost the same way as air-cooled chillers. The difference is that they remove heat from chilled water by exhausting the heat to a second, isolated water line called the condenser water line. The condenser water flows through the chiller and picks up heat. The condenser water then returns to the cooling tower. The cooling tower is almost always located outside of the building and removes heat from the condenser water by evaporating some of the condenser water into the atmosphere. As some of the condenser water evaporates, heat is removed from the condenser water, and the cool condenser water flows back to the chiller. This process is then repeated all over again. Water-cooled chiller systems are very energy efficient. However, due to their complexity and many different parts, they are often more expensive to install and maintain. For this reason, you will usually only find them in large buildings. This is because the energy savings outweigh the cost of installing and maintaining the system.
• Air Cooled Chillers: An air cooled chiller absorbs heat from water and transfers it into the air and is used where discharge is not a problem. Heat from circulating chilled water is absorbed in the evaporator. In the condenser, the refrigerant condenses, releasing the heat into the air. Air-cooled chillers are almost always located outside of a building and remove heat from the chilled water by exhausting the heat directly to the surrounding air. Air-cooled chillers exhaust heat from the condenser coil. As warm refrigerant passes through the condenser coil, the outside air blows over the condenser coil and removes heat from the refrigerant. The refrigerant then passes through an expansion valve, where it rapidly cools and goes through the evaporator, where it cools the chilled water. This process is then repeated all over again.
• Screw Chillers: Screw chillers can be water or air cooled and use a helical rotor to move and compress the refrigerant vapors. Screw compressors are used for both water cooled and air cooled chillers. With water cooled type the compressor is on top of the chiller and with air cooled type the compressors is under the chiller. Indoor water cooled chillers will often be insulated whereas air cooled may not be. This type takes the refrigerant off of the evaporator and passes through into the compressor. Inside the compressor are two interconnecting screws. The refrigerant will enter into a void between the two screws, but as the screws rotates they push the socket of refrigerant further into the compressor and squeeze it into a small space. The refrigerant will exit at high pressure high temperature and flows to the expansion valve. Although centrifugal compressors are more efficient at full load, rotary-screw compressors offer the best performance during part-load operation. Since the cooling load on a building varies throughout the year, rotary-screw compressors tend to achieve the lowest operating cost overall, even if centrifugal compressors are more efficient at full load. The main limiting factor of rotary-screw compressors is their high price. If the application does not provide opportunities to take advantage of their capacity control and their superior part-load efficiency, another type of compressor can be considered.

• Scroll Chillers: Scroll chillers have a set of scrolls that are used to compress the refrigerant and operate quieter and more efficiently. Since they are environmentally friendly, they are becoming more popular. The scroll compressor is used mostly on air cooled chillers but you can also find them on water cooled. Usually one compressor isn’t enough to meet the cooling load so several will be joined together in a bank. In the example above the blue cylinders under the chiller are the compressors which are joined to form a bank. With these type of compressors, the refrigerant usually enters via the bottom and is fed into the compressor discs. One disc will be stationary whilst the other is rotated to compress the refrigerant into a tighter space. The refrigerant is forced around the spiral as the disc moves which causes it to compress, it will then leave via the top and head to the expansion valve.
• Centrifugal Chillers: Centrifugal chillers use compression to convert kinetic energy into static energy to increase the temperature and pressure of the refrigerant. Impeller blades pull in the refrigerant and compress it. The centrifugal type compressor is quite easy to spot as the compressor is above the chiller with a large volute shaped pipe curling around into the condenser. The refrigerant flows in through the suction line, hits into the the centre of the impeller where it will be directed by the blades. The blades rotate and that imparts an angular velocity onto the particles of the refrigerant. This angular velocity makes the refrigerant particles fly out at high velocity, in all directions, and collects in the volute (the outer curl) where it increases in pressure from the kinetic energy, it then passes down into the condenser. This type of compressor/chiller is a very common set up for a central plant in large buildings. If a reciprocating compressor can be compared to a car engine, this type is comparable to a water pump because it also uses an impeller, with the difference that it operates with refrigerant. These compressors are also available in hermetic and open models, where the open construction offers higher efficiency. At rated load, centrifugal compressors are more efficient than both reciprocating and rotary-screw compressors. They also offer a compact construction and are available in a wide range of cooling capacities. Capacity control is normally achieved with inlet vanes that increase or decrease the flow of refrigerant into the impeller. Despite their superior efficiency at full load, centrifugal compressors suffer from a drastic loss of efficiency at part-load. At very low cooling loads they are rendered unable to operate, due to a phenomenon called surging: refrigerant that is already compressed flows back into the impeller, disrupting its operation. Centrifugal compressors have the least number of moving parts among the three main compressor types, which means there are less components to service. However, in most cases the impeller must be factory-ordered if fails; suppliers rarely keep stock because it’s a highly specialized component. Balancing and vibration must also be checked frequently to avoid loss of performance and premature component failure.

• Absorption Chillers: In an absorption chiller, a generator uses steam or hot water to change the refrigerant into a vapor, which moves to the condenser where the vapor is sent back to the absorber. The refrigerant vapor is absorbed by a solution, which condenses into a vapor to release heat.
• Reciprocating Chillers: Reciprocating chillers use pistons and a chamber to create pressure in the refrigerant. They can have sealed or open construction with sealed units having all of the components sealed in a single unit. Since reciprocating units function like an automobile engine, they require regular maintenance. These type are becoming less common because newer, more efficient, technology has been replacing it so these are slowly being phased out in commercial refrigeration at least. It’s still quite popular in industrial refrigeration. These are very strong and reliable compressors which seem to just work forever with the right maintenance. There are a lot of moving parts though so they can be expensive to operate. In reciprocating types the refrigerant will often pass over the electrical motor to proving cooling to the electrical coils and then head into the compression chamber. The compression chamber is simply a number of piston and chambers which the refrigerant will flow into. The piston is on a crank which moves it up and down. As it moves it will compress the refrigerant into the chamber and at a timed interval the refrigerant will exit at a high pressure. For a given cooling capacity, reciprocating compressors have a lower upfront cost than centrifugal and rotary-screw compressors. They also offer design flexibility, allowing multiple units to be installed together to serve variable cooling loads – individual units are activated or deactivated as needed. The main limitations of reciprocating compressors are their lower energy efficiency and their demanding maintenance, compared with other types of compressors. Also, although these compressors can achieve good capacity control when multiple units are used

• Explosion Proof Chillers: Explosion proof chillers are designed for heavy duty use and must follow specific National Fire Protection Agency (NFPA) guidelines in their construction. They have a special reinforced structure for protection against flammable materials and have to be specially ordered. The main purpose of explosion proof chillers is for the protection and safety of workers. Explosion proof chillers operate on the same principles as a regular chiller but with added reinforced protection.
• Low Temperature Chillers: Low temperature chillers are for industries that operate below freezing and require chillers that can produce temperatures at – 40° F. They are used for ice rinks, petrochemical cooling, chemical, extraction, medical, pharmaceutical and food processing as well as product testing labs.
• Evaporative Chillers: An evaporative chiller uses the power of evaporation to cool air. When water evaporates, it becomes a gas. High energy particles leave causing the temperature of the surrounding air to radically drop. This process can be felt when air is misted into a room. An evaporative chiller takes the natural process of evaporation and enhances it through the use of technology. The process of an evaporative chiller requires the use of a reservoir of water, a fan, and thick pads. The fan draws in hot air, which crosses through the thick pads that absorb water from the reservoir. As the hot air passes through the pads, the water on the surface evaporates and causes the air temperature to drop by nearly 20 degrees.

Chiller Condenser Types

What is a condenser? How does it work? How many types of condensers are there? A condenser is a cooling device. The condenser is located after the compressor and before the expansion valve. The purpose of the condenser is to remove heat from the refrigerant which was picked up in the evaporator. There are two main types of condensers, Air cooled and Water cooled. Water cooled condensers will repetitively cycle “Condenser water” between the cooling tower and the condenser, the hot refrigerant which enters the condenser from the compressor, will transfer its heat into this water which is transported up to the cooling tower and rejected from the building. The refrigerant and the water do not mix they are kept separated by a pipe wall, the water flows inside the pipe and the refrigerant flows on the outside.
Condensers on air cooled chillers work slightly differently, they do not use a cooling tower but instead blow air across the exposed condenser pipes with the refrigerant flowing this time on the inside of the tube. The device liquefies gas by cooling it. Every refrigeration system uses condensers to condense steam or vapor to bring about a cooling effect. A condenser works in three phases, namely:

• De superheating
• Condensation
• Sub-cooling 

Usually, the vapor is super pressurized and superheated in the compressor and evaporator before it enters the condenser. The condenser then ejects the heat (desuperheats) from the vapor and liquefies it. More heat is lost in the condensation phase, and most of the vapor turns to liquid. The sub-cooling state is there to ensure that no liquid refrigerant turns into vapor even with temperatures rises.

• Air-Cooled Condensers : These condensers follow a simple design and use the standard airflow in managing their cooling. The air-cooled condensers are mainly found in small units such as water coolers, household refrigerators, window air-conditioners, deep freezers, small packaged air-conditioners, split air-conditioners, etc. The cooling load in these units is relatively small, and the refrigerant quantity used is small.  Some refer to the air-cooled condensers as coiled condensers since they are made of aluminum or copper coil. Air-cooled condensers are further subdivided into two categories: the forced convection types and the natural convection types. The forced convection condensers use a motor-operated fan to blow air in the condenser coil. In contrast, the natural convection types rely on the natural airflow depending on the condenser coil’s temperature.
• Water-Cooled Condensers : the water-cooled condensers use water as the fluid to remove heat from the refrigerant. They are the eco-friendly variation of the standard air-cooled condensers. These condensers are mainly found in central air-conditioning, big packaged air-conditioners, large refrigerating plants, etc. They are applicable in areas where the cooling loads are very high, and large quantities of refrigerant flow via the condenser. Like the air-cooled condensers, the water-cooled condensers come in different types, including double-type/tube-in-tube, Shell & tube type, and shell & coil type. The working mechanism is almost the same in all of them. One side of the piping flows the refrigerant, and water flows through the other piping, thus cooling and condensing the refrigerant. Water-cooled Condensers may be best known for their quick and hassle-free installation, long-lasting performance, quiet operation, and energy efficiency.
  • Shell and Tube Water Cooled Condenser : Shell-and-tube is one of the most common types of water-cooled condensers. This efficient heat exchanger is easy to clean and repair, contributing to a hard-working and cost-effective water-cooled chiller. The construction materials of a shell-and-tube condenser often include a carbon or stainless-steel shell with welded grooved end plates and straight copper water tubes positioned for a vapor-tight fit. During operation, the compressor releases hot refrigerant into the shell. At the same time, water flows through the tubes, leaving the refrigerant inside the shell. Once the water-cooled chiller’s refrigerant hits the water tubes, the vapor begins its liquid conversion before releasing the newly condensed liquid refrigerant through the cylindrical shell, where it will make its journey to the expansion valve.
  • Pros:
    • Easy to clean
    • Easy to locate and plug tube leaks
    • Ideal for pump-down and low ambient environments
    • Thick tube walls for rugged mechanical construction
    • Lower probability of fouling and scaling
    • Less susceptible to freeze damage
  • Cons:
    • May not be suitable for applications with limited space
    • More expensive than brazed plate water-cooled condensers
    • Greater surface area can reduce thermal efficiency
  • Brazed plate Water Cooled Condenser : Brazed plate is another type of water-cooled condenser you’ll find in packaged water-cooled chillers. It’s best known for its cost-effectiveness and corrosion-resistant and enhanced heat transfer properties. Brazed plate water-cooled condensers feature a non-ferrous construction that resists rusting and corrosion. Alternating metal plates are held together with a copper-based brazing material and feature an embossed pattern to create a fluid channel. During a water-cooled chiller’s operation, cold fluid makes contact with one side of the plates and not the other. This water-cooled design helps generate highly efficient heat transfer between process and refrigerant fluids.
    • Pros:
      • Compact design is ideal for limited space environments
      • Non-ferrous construction resists corrosion and rust
      • Enhanced heat transfer, thanks to the small passages that encourage a turbulent flow
      • Economical and cost-effective
    • Cons:
      • Small passages can cause elevated water side pressure drops
      • Small passages are susceptible to plugging and fouling
      • Brazing material is subject to corrosion
  • Coaxial tube-in-tube Water Cooled Condenser : Coaxial tube-in-tube is the third type of condenser in a water-cooled chiller. It earns its name because the tubes are coiled around the same axis. Excellent anti-fouling characteristics make these high-performance and compact heat exchangers stand out from other condenser types. Coaxial tube-in-tube is the third type of condenser in a water-cooled chiller. It earns its name because the tubes are coiled around the same axis. Excellent anti-fouling characteristics make these high-performance and compact heat exchangers stand out from other condenser types.
• Air-Water (Evaporative) Cooled Condensers : This condenser is a combination of both water and air condensers. Water sprays onto the coils; it evaporates, causing a temperature drop within the coils. The refrigerant within the coil condenses and cools down. In addition, cool air is also released from the condenser’s bottom, blowing across the coils. In this type of condenser, both air and water work in tandem to cool and liquefy the coil’s refrigerant. These condensers are eco-friendly and are ideal for commercial HVAC systems. The air-water condensers are cheaper compared to the water-cooled condensers. They also offer an effective cooling solution in areas with a low water supply.
  • Pros:
    • Highly effective operation
    • Compact design
    • Excellent anti-fouling characteristics
    • Ideal for high temperature, high pressure, and low-flow applications
  • Cons:
    • Can be cost more than other water-cooled condensers
    • It’s not feasible to clean the water side, making chemical cleaning the only solution should fouling occur
    • May not be suitable for large industrial process cooling applications

Chiller Evaporator Types :

The evaporator is that part of the refrigeration system where the liquid refrigerant is evaporated. It is sometimes called the cooling coil, unit cooler, freezer coil, liquid coolers, etc. As the name, it is a part of the system where the liquid refrigerant is changed into a vapour by the absorption of heat is an evaporator. It is fitted on low pressure side in the circuit. Evaporators are classified as flooded type and dry type depending upon whether liquid refrigerant covers all heat transfer surfaces or some portion is having gas vapour being superheated. The evaporator with thermostatic expansion valve can be designed dry evaporator whereas evaporator with float valve will be flooded type. There are many types of evaporator, these are classified as: 

• According to the type of construction :
  • Bare tube coil evaporator.
  • Finned tube or extended surface type evaporator.
  • Plate type evaporator.
  • Shell and tube evaporator. 
  • Shell and coil evaporator.
  • Tube in tube evaporator.
• According to the manner in which liquid refrigerant is fed:
  • Flooded evaporator
  • Dry-expansion evaporator.
• According to mode of the heat transfer:
  • Natural convection evaporator
  • Forced convection evaporator.
• According to operating condition:
  • Frosting evaporator.
  • Non-frosting evaporator.
  • Defrosting evaporator.
• According to expansion :
  • Direct expansion evaporator.
  • Indirect expansion evaporator.

Shell and Tube type Evaporator :

• The shell and tube evaporator is similar as shell and tube condenser. It consist of shell, tube sheets and tubes, waler boxes and refrigerant connection. In the smaller size the shell may be standard pipe but welded shell are used for large size.
• Two pass horizontal shell and tube evaporator is equipped with enclosed water box and mounted in horizontal position. 
• The tube sheet usually 1 inch thick are welded to the shell and drilled to receive tube.
• The tubes are inserted through their respective tube sheet hole and welded to provide gas tight joint. 
• The refrigerant vapour from the compressor expanded in the tube and chilled water is circulated through the shell.
• If the evaporator is operated flooded then water is circulated through the tube and liquid refrigerant is circulated through the shell 
• The height of liquid refrigerant in the shell is controlled by float valve.

Shell and Tube type Evaporator :

• The shell and tube evaporator is similar as shell and tube condenser. It consist of shell, tube sheets and tubes, waler boxes and refrigerant connection. In the smaller size the shell may be standard pipe but welded shell are used for large size.
• Two pass horizontal shell and tube evaporator is equipped with enclosed water box and mounted in horizontal position. 
• The tube sheet usually 1 inch thick are welded to the shell and drilled to receive tube.
• The tubes are inserted through their respective tube sheet hole and welded to provide gas tight joint. 
• The refrigerant vapour from the compressor expanded in the tube and chilled water is circulated through the shell.
• If the evaporator is operated flooded then water is circulated through the tube and liquid refrigerant is circulated through the shell 
• The height of liquid refrigerant in the shell is controlled by float valve.

Extended Surface Evaporator (Finned Evaporator): Finned coils are bare-tube coils upon which metal plate or fins have been installed. The fins servicing as secondary heat absorbing surfaces, have the effect of increasing decide evaporator, thereby improving its efficiency for cooling air and other gases. With bare-tube evaporators, much of the air that circulates over the coil passes through the open space between the tubes and does not come in contact with coil surface. When fins are added to a coil, the fins extended out into the open space between the tubes and acts as a heat collector. These fins remove heat from the portion of the air that would not ordinarily come in contact with the tube surface. In some cases fins are soldered directly to the base tube. Fin size and spacing depend upon the particular type of application for which coil is designed. The size of tube determine the size of fin. Small tube require small fins. Fin spacing varies from I to 14 fins per inch depending upon operating temperature of the coil. The evaporator designed for low temperature application must have wide fin spacing in order to minimize the danger of restricting air circulation. Excessive finning may actually reduces the evaporator capacity by restricting the air circulation over the coil unnecessarily  These evaporator are best suitable for air conditioning application where refrigerator temperature above 0°C because of rapid cooling rate. It is compact, occupy less space than bare tube or plate evaporator for same capacity. This evaporator is a finned coil type because of rapid heat transfer rate during the off cycle. It also maintain a relative humidity of about 90 to 95%. It requires larger evaporator surface area to make up this Loss. It has a disadvantage when installation at the top of evaporator, may defrost and moisture flows down the evaporator surface. This moisture will not get sufficient time to escape before compressor lowers the temperature of the evaporator to -7°C to – 6°C range when this occurs, the frost accumulate at the lower part of the evaporator which results in block of fins gap and so free circulation of air around the coil these reduces the effectiveness of evaporator.

Bare-Tube Evaporator :

• It is simple in construction hence it is very easy to clean and defrost.
• These are constructed from steel pipe or copper tubing. 
• Steel pipe used for larger evaporator and are applicable for ammonia refrigerant.
• The copper tubing is utilized for smaller evaporator for use with refrigerant other than ammonia. 
• Bare-tube coil are available in a number of size and shape.
• The common shape for bare-tube coils are flat zig-zag.
• Spiral bare tube coil are often employed for liquid cooling.
• This evaporator provides less surface contact as compared to other. 
• The contact surface area may be increased by extending the length of the tube.
• If the tube is too long, the liquid refrigerant completely vaporized carly leading to excessive superheating at the outlet. 
• This also causes greater pressure drop between inlet and outlet of the evaporator. This reduce suction pressure to compressor.
• If diameter of tube is large, refrigerant velocity is low and specific volume of refrigerant will be greater, to the surface of the tube to allow complete vaporization. This may allow liquid refrigerant to suction line of compressor with possible damage to the compressor 
• If diameter is too small, causes pressure drop due to friction and reduce efficiency.

Plate Sheet Type Evaporator :

• These are of several types. Some are constructed of two flat sheets of metal so embossed and welded together to provide path for refrigerant flow between two sheets.
• This type is widely used in household refrigerators and home freezers because of easy to clean, economical to manufacture.
• Another type of evaporator consists of formed coil Installed between two metal plates that are welded together at the edges.
• This provides good thermal contact between the welded plates and coil carrying the refrigerant.
• Plate type evaporator may be used singly or in bank.
• The plate can be grouped together for ceiling mounting in holding room, freezer.
• The plate may be manifold for parallel flow or series flow of refrigerant. 
• These are used for smaller capacity equipment required for peak load.

Flooded Evaporator :

• In a flooded evaporator, a constant liquid refrigerant level is always maintained.
• A float control valve is used as an expansion device which maintains constant liquid level in the evaporator.
• A accumulator placed between the evaporator and the compressor. It catches any liquid droplet that passes over and drains it back to the bottom of the evaporator.
• Due to the heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil vaporizes and thus liquid level falls down.
• An accumulator supplies more liquid to evaporator to maintain the liquid level in evaporator. So the level in the accumulator falls down and so the float falls down and open float valve. Hence liquid level from receiver is entered into the accumulator. As liquid level in the accumulator rise float rise up and float valve closed. 
• As the refrigerant absorb heat from warmer substance, the vapour formed by vaporizing the liquid in the coil being lighter, rises up and passes on to the top of the accumulator from where it is supplied to the suction side of the compressor 

Flooded Evaporator :

• In a flooded evaporator, A constant liquid refrigerant level is always maintained.
• A float control valve is used as an expansion device which maintains constant liquid level in the evaporator.
• A accumulator placed between the evaporator and the compressor. It catches any liquid droplet that passes over and drains it back to the bottom of the evaporator.
• Due to the heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil vaporizes and thus liquid level falls down.
• An accumulator supplies more liquid to evaporator to maintain the liquid level in evaporator. So the level in the accumulator falls down and so the float falls down and open float valve. Hence liquid level from receiver is entered into the accumulator. As liquid level in the accumulator rise float rise up and float valve closed. 
• As the refrigerant absorb heat from warmer substance, the vapour formed by vaporizing the liquid in the coil being lighter, rises up and passes on to the top of the accumulator from where it is supplied to the suction side of the compressor 

Flooded Evaporator :

• In a flooded evaporator, A constant liquid refrigerant level is always maintained.
• A float control valve is used as an expansion device which maintains constant liquid level in the evaporator.
• A accumulator placed between the evaporator and the compressor. It catches any liquid droplet that passes over and drains it back to the bottom of the evaporator.
• Due to the heat supplied by the substance to be cooled, the liquid refrigerant in the evaporator coil vaporizes and thus liquid level falls down.
• An accumulator supplies more liquid to evaporator to maintain the liquid level in evaporator. So the level in the accumulator falls down and so the float falls down and open float valve. Hence liquid level from receiver is entered into the accumulator. As liquid level in the accumulator rise float rise up and float valve closed. 
• As the refrigerant absorb heat from warmer substance, the vapour formed by vaporizing the liquid in the coil being lighter, rises up and passes on to the top of the accumulator from where it is supplied to the suction side of the compressor 
• Dry-expansion coolers are generally used halo-carbon refrigerant.

Chiller Maintenance

• Condenser: Heat transfer is a major part of a chiller‘s operation. Condenser coils can become clogged or have free air passage.
• Refrigerant: A chiller‘s ability to perform properly is highly dependent on the refrigerant. Improperly charged refrigerant can severely impact the chiller‘s performance.
• Water: Water used with cooling towers has to meet the parameters for proper water flow. Debris, dirt, solids, and contaminants can interfere with water flow and be detrimental to the chiller.
• Reservoir Check: For the best performance from a chiller, all of its reservoirs should be checked to be sure they have an adequate supply of fluids.
• Temperature Check: Chillers operate at their optimum at 50° F. Unmonitored temperature changes can be harmful to the operation of the chiller. For best results, a regular examination of the glycol inlet and outlet temperatures help in catching possible problems.
• Cleaning: All equipment collects dirt and dust in the manufacturing process. For peak efficiency, the exposed parts of a chiller should be regularly cleaned. Filters should be changed to avoid clogging.

Chiller Oil Lubrication Circuit

How does a chiller oil lubrication system work? Chillers need to force oil around some of its internal components to provide lubrication and remove heat caused by friction. In some models the oil is used to control and adjust the movement of the vane guides which control the cooling capacity. The oil must be forced onto the surfaces of critical components such as the bearings and drive transmission to protect it from mechanical faults and prolong the life of the machine. If the flow of lubrication oil stopped, the machine would destroy itself because of the excessive heat that would build up. Luckily most chillers have inbuilt safety controls to prevent such scenario.

A small submersible oil pump forces the oil around the system and sites within the sealed vessel. An electrical heating element also sits within the vessel to ensure oil it kept at the correct temperature, it will turn on and warm the oil if the temperature sensor detects that it is too low. The oil pump forces the oil to mix into a return oil stream where it then enters the heat exchanger, usually of plate or shell and tube type design, where either refrigerant or cooling water also enters the heat exchanger to remove any unwanted heat to maintain a specified supply oil temperature.

The oil is then sent up to the top of the compressor and will usually pass through an oil filter before entering a small oil reservoir. The filter helps to prevent foreign particles entering the moving components as these will damage the machine. The reservoir provides an emergency supply. If the chiller suddenly lost electrical power, the oil pump would be unable to provide lubrication oil while the machine slowed down. The reservoir therefore provides a gravity fed supply of oil during this period until the rotating components have came to a complete stop. From the reservoir the oil is distributed to a few key components. In one stream the oil is sprayed as a fine mist over the drive transmission gears which ensures an even coat of oil. In another stream the oil is forced at high pressure onto the bearings and thrust bearings. It needs to be under high pressure to ensure it enters into all the small gaps and covers all the surfaces.

The oil from the first stream, drive transmission, is then usually collected in a reservoir and sent back to the vessel so the sump pump can continue to force oil around the system. The oil from the second stream, bearings, is typically collected and sent straight back to the heat exchanger as it will be much hotter so needs to be cooled.

Water Cooled Chiller Maintenance List

• Check leaving water temperature, temperature and load limit set point.
• Check for water and oil leaks. Rectify and report to Site Manager.
• Check for excessive noise and vibration and report to Site Manager.
• Check for excessive temperatures and pressures.
• Check operation of oil heater.
• Check refrigerant sight glass and record indicated condition and charge of refrigerant.
• Check chiller oil levels. If low, determine and correct reason for oil loss before adding oil. Determine existing oil type and grade, prior to addition to oil. Add only identical oil type and grade and record quantity of oil added.
• Check purge unit and record purge count. Investigate further if purge system operates excessively.
• Leak test all refrigerant circuits in accordance with HB40.1.
  • Rectify minor leaks, immediately advise Site Manager of any leakages found.
  • With compressor running record:
    • Motor amps
    • Discharge pressure
    • Suction pressure
    • Oil pressure
    • Chilled water flow and return temperature
    • Condenser water flow and return temperature
    • Hours run and number of starts
    • Water pressure drop across evaporator
    • Water pressure drop across condenser
• Check operation of all safety and operating controls including HP, LP and low oil pressure controls. Maintain set point strictly in accordance with chiller set manufactures specifications.
• Check operation and calibration of chilled water low temperature thermostat and flow switches.
• Clean chiller equipment surfaces and immediate plant area.
• Remove both condenser ends. Advise Site Manager when system is open for inspection and report conditions of tubes, tube plates and water box covers. Seek approval to clean tubes before proceeding. Mechanically clean condenser tube bundles with mechanically driven brush or honing device. Clean water boxes and check water box epoxy coating, repair if necessary. Seek approval from Site Manager before reassembling.
• Remove used oil sample from chiller and submit to test laboratory for analysis. The chiller must be in operation when obtaining an oil sample. (Collection of stagnant oil for testing will result in erroneous readings). Oil sample bottle must be suitably identified with date, machine operating hours and correct description of oil type, manufacturer and sampling technician. The bottle itself must be of a suitable type, similar to those supplied by oil analysis companies. Provide Site Manager with a copy of the sample test results.
• Compressor oil replacement should only be effected if oil test results show abnormal oil conditions (i.e. Results are outside the manufacturer’s specified limits). If changing compressor oil first obtain laboratory test of new oil sample prior to filling machine and provide the Site Manager with a copy of the test results.

Air Cooled Chiller Maintenance List

• Check for water and oil leaks. Rectify and report to Site Manager.
• Check for excessive noise and vibration and report to Site Manager.
• Check for excessive temperatures and pressures.
• Check operation of crank case heaters and heater relays.
• Check liquid line, sight glass and moisture indicators and record indicated condition and charge of refrigerant.
• Check compressor oil levels, for compressors off line check and record crankcase oil levels, record oil level of operating compressors if in operation for at least 30 minutes, determine and correct reason for oil loss before adding oil, determine
• existing oil type and grade, prior to addition to oil, add only identical oil type and grade and record quantity of oil added.
• Check all set points for proper setting and function
• Complete overall visual inspection to be sure all equipment is operating
• Check compressor oil level, adjust if necessary.
• Check operation of crankcase heater
• Check all safety devices and controls
• Check temperature controls
• Check refrigerant charge and circuit, especially for leaks
• Check condition and operation of all indicator lamps
• Check operation of condenser fans
• Record all readings and results in log book
• With compressor running record:
  • Number loaded cylinders
  • Compressor discharge pressure
  • Compressor suction pressure
  • Compressor oil pressure
  • Chilled water flow and return temperature
  • Condenser water flow and return temperature
  • Hours run
  • Water pressure drop across evaporator
  • Water pressure drop across condenser
• Check condition of condenser fins and tubing especially for cleanliness and any obstructions
• Motor
  • Check condition and operation of motor including:
  • Record motor current draw at full operating load and compare with rated output
  • Starter contacts
  • Overloads, Electrical connections for security, tightness,
  • contact and corrosion
  • Check plant and enclosure for cleanliness

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