There are six basic types of cooling systems that you can choose from to meet the cooling needs of your load. Each one has its strengths and weaknesses. This article was written to identify the different types of cooling systems and identify their strengths and weaknesses so that you can make an informed choice based on your needs.
There are six basic types of liquid cooling systems:
- Closed-loop dry system
- Closed-loop dry system with trim cooling
- Open-loop evaporative system
- Closed-loop evaporative system
- Chilled water system
Liquid-to-Liquid Cooling Systems
The simplest of these systems is a liquid-to-liquid cooling system. In this type of system your plant has an abundance of some type of cooling liquid already available but you do not want to provide this coolant to the compressor. For example: you have well water available but you do not want to put the well water through your new compressor because the water quality is very poor (lots of dissolved solids like iron and calcium etc.), and you have had trouble with the well water fouling your heat exchanger/s in the past.
A liquid-to-liquid cooling system is an ideal fit for this situation. It uses the well water on one side of an intermediate heat exchanger and a coolant such as glycol and water on the other side of the intermediate heat exchanger in a closed loop to cool the compressor. The heat is exchanged through the intermediate heat exchanger without fouling the heat exchanger/s. Fouling of the intermediate heat exchanger will likely happen on the well-water side, however, if the intermediate heat exchanger is selected properly it can be taken apart easily and cleaned. The most common intermediate heat exchangers are either plate and frame or shell and tube type. Coolant temperatures of 5 degrees above the plant cooling “water” are possible with a liquid-to-liquid type system. In the well water example above if the well water is available at 55 F the liquid to liquid cooling system is capable of supplying 60 F coolant to the load.
The strength of a liquid-to-liquid cooling system is that it is relatively inexpensive to purchase and install. The components can be installed inside or outside. The system is inexpensive to operate with only the closed loop pump using any additional energy. Maintenance is relatively simple demanding only a periodic inspection, lubrication, and cleaning of the heat exchanger as necessary.
Liquid-to-Liquid Cooling Systems
Weaknesses of the liquid-to-liquid cooling system include periodic downtime of the cooling system for cleaning. This can be offset by installing a standby intermediate heat exchanger that is put into service while the primary intermediate heat exchanger is cleaned. A standby heat exchanger adds additional cost but allows for continuous operation of the cooled load while cleaning is accomplished. This system requires a regulated supply of plant coolant like the well water example above for proper cooling of the load. There can be times that the cooled load does not operate at maximum capacity and the plant primary cooling “water” must be regulated to insure the load is not over or under cooled.
Closed-Loop Dry Cooling Systems
A closed-loop dry cooling system is very much like the radiator in your car. The system uses an air-cooled fluid cooler to transfer the heat from the closed-loop coolant fluid pumped through rows of finned tubes that have ambient air blown/drawn across them. The basic components to a closed-loop dry cooling system are the fluid cooler, which contains the air to liquid heat exchanger with the fan/s, the pump and control skid, the coolant, and the field installed system piping. The closed-loop dry cooling system fluid cooler will be located outside and use the ambient air to reject the heat. Coolant temperatures of 5 to 10 F above the ambient dry bulb temperature are possible with a closed-loop dry cooling system. The system is relatively inexpensive to operate with only the coolant pump and the fluid cooler fan/s using energy. The fan/s are thermostatically controlled to regulate the temperature of the cooling fluid so that the load is not over or under-cooled. Periodic cleaning of the fluid cooler may be necessary due to dirty atmospheric conditions at the site location. Fouling of the fluid cooler is typically caused by dirt, leaves, cotton-wood seeds, etc.
Closed-loop Dry Cooling Systems
The strength of a closed-loop dry cooling system is that the unit is very simple and relatively easy to install. The energy requirements are relatively low and it is easily controlled. Maintenance is normally low requiring only periodic inspection, lubrication, and testing of the fluid.
The weakness of a closed-loop dry cooling system is that it is dependent on the atmospheric dry bulb. For example, if your location’s design dry bulb is 100 F in the summer and your equipment requires 90 F coolant; at best the system can only supply around 105 to 110 F coolant to the load. In this case you would need supplemental cooling to get the coolant temperatures down to 90 F.
The closed-loop dry cooling system also requires free clear air to work efficiently. This means that the fluid cooler must be placed in a location that is not affected by the prevailing winds, not too close to a building that will allow the warm exhaust air from the fluid cooler to be recirculated back to the fluid cooler, and finally not in a location that has heavy concentrations of dust, dirt, leaves, seeds, etc.
Many times the best location for the fluid cooler is on the roof. Since the fluid cooler is located outside the coolant must also have a concentration of some type of glycol to prevent freezing if your location has a design dry bulb in the winter that dips below freezing. If the location is very cold, the concentration of glycol may need to be significant to prevent freezing. Glycol concentrations as they increase begin to reduce the rate of heat transfer. For example, if you need 50% ethylene glycol concentration with water the heat exchanger equipment and the flow/pressure of the coolant will need to increase to adjust for the glycol concentration. Larger fluid coolers and pumps will increase the cost of the system over those with lesser concentrations of Glycol/water. This cannot be prevented in colder climates.
Closed-loop Dry System with Trim Cooling
A closed-loop dry system with a trim cooler is the same as the closed-loop dry system but adds a supplemental fluid cooler. This system is typically used in a location that has too high of a dry bulb in the summer to provide the proper coolant temperature to the load. With an added liquid-to-liquid trim cooler the customer can use a water source to trim the temperature to the desired set point. Many times closed-loop dry system with a trim cooler are used to reduce the reliance on city water as a coolant. City water is becoming expensive to buy and to dispose of. These systems may be employed to completely eliminate the city water usage most months in a year, thus reducing the plant’s operating costs. The system must have a supply of free clear air and a regulated supply of plant coolant or city water as with a liquid-to-liquid cooling system.
The strength of the closed-loop dry system with a trim cooler is that it can provide coolant temperatures below that of a closed-loop dry system alone. The system will reduce the amount of plant/city water usage during the colder months.
The weaknesses of the closed-loop dry system with a trim cooler include all of those listed for the closed-loop dry system. Also, it now requires some secondary coolant during warmer times of the year. Additional piping will be required for the trim coolant to/from the skid. Both the trim cooler and the air cooled fluid cooler will require periodic maintenance and cleaning.
Open-loop Evaporative Cooling Systems
The next system, an open-loop evaporative cooling system is completely different than the first three listed above. This system has the ability to use the design wet bulb as the basis for the outlet temperature of the cooling water. For example if the design dry bulb for the location is 95 F and the design wet bulb is 75 F, the system can provide approximately 82 F water to the load.
The open-loop evaporative cooling system cascades water through the honeycomb PVC fill material in the tower along with ambient air blown or drawn through the fill to evaporate the water. During the evaporation, the remaining water is cooled to as close as 7 F or higher above the wet bulb temperature. The evaporated water is replaced with some type of make-up water system like a float valve. The remaining water and the make-up water are collected in a basin and then pumped to the load and the cycle repeats. On average an open-loop evaporative cooling system requires 4 GPM of make up and blow down water per 1,000,000 Btu/hr of heat rejected.
Open-loop Evaporative Cooling Systems
The advantage of this system is that the equipment is typically inexpensive. The systems can be simple to employ in warmer climates but may require more controls in colder climates.
The weaknesses of this type of system are that they normally require an extensive water treatment system. The water treatment system uses expendable chemicals to keep the calcium and dissolved minerals in suspension. The chemical treatment is necessary to ensure that the cooling tower, piping, and heat exchangers do not become fouled. An inherent issue with the open tower evaporative system is that the water that flows through the tower is also the heat transfer fluid that is pumped through the load. This water comes in contact with the dirty atmosphere. It picks up pollutants such as dust, vegetation, etc. These contaminates end up in the heat exchangers and piping and can cause significant maintenance issues.
Open towers can have control issues in the winter months. They are designed to run at full load. They do not always perform well under part-loading in very cold climates. If the basin is part of the tower, a heater is required for cold weather operation to keep the basin water from freezing when the load is not present. The piping will normally require insulation and heat trace in cold climates to prevent freezing. A drain will be required for blow-down of the water to keep the conductivity in check from the constant evaporating and concentrating of the dissolved solids. Make-up water is continually required from external source such as city water or treated well water, etc. Biological control of bacteria, slime, and mold are major concerns for proper operation of an open evaporative tower system.
Closed-loop Evaporative Cooling Systems
A closed-loop evaporative system is a hybrid system. The closed loop evaporative system is an open tower with a closed-loop heat exchanger built into the tower. The tower water stays outside in the tower and does not circulate through the coolant piping. The coolant piping is a closed loop, with a glycol/water solution flowing from the tower to the load and back. The separate tower water is pumped from the basin to the top of the tower and sprays across the heat exchanger (normally an array of tubes) with air blown or drawn through the tower across the heat exchanger where evaporation of the water transfers the heat from the closed coolant loop to the ambient air. The remaining tower water falls to the basin where it is again pumped up to the top of the tower and repeats the process. The closed-loop evaporative system tower water requires make-up water, chemical treatment, a drain, cold weather basin heater, and blow-down just like the open-loop evaporative system discussed above.
Closed-loop Evaporative Cooling Systems
The advantage of the closed-loop evaporative system is that it can deliver closed loop coolant to the load at approximately 7 to 10 F above the wet bulb temperature. The closed-loop coolant remains free of contaminates and allows the equipment heat exchanger and piping to remain clean. Any contaminates from the atmosphere will stay outside with the tower. Fewer water treatment chemicals will be used because they are only treating the open water in the tower and not the coolant in the piping and system heat exchangers.
The drawbacks of a closed-loop evaporative system are that you will need water treatment, blow-down, and make-up water for the tower water side of the system. The system will require a drain and heat-traced and insulated piping for cold weather applications. There is a basin heater required to prevent freezing of the basin in cold weather off-time operation. The system requires an additional pump connected to the tower which circulates the basin water.
Chilled Water Cooling Systems
The last type of cooling system we will discuss is the chilled water system. A chiller normally has a mechanical compression device that converts energy into compressed refrigerant by using some type of compressor. The compressed refrigerant is piped to a condenser that rejects the heat from the refrigerant to the atmosphere or some type of liquid coolant. The compressed refrigerant changes state from a gas to a liquid in the condenser and is piped to an evaporator where it is metered or expanded in the evaporator. The expansion of the high pressure liquid refrigeration reduces the temperature of the evaporator. The liquid to be cooled is pumped through the evaporator heat exchanger and heat is transferred to the refrigerant. The low pressure vapor is carried back to the compressor and the cycle begins again for the refrigerant. The coolant flows from the evaporator heat exchanger to the load where the heat is transferred to the coolant in the load heat exchanger and then returns back to the evaporator to repeat the cycle.
Chilled Water Cooling Systems
The strengths of a chiller are that it can produce coolant temperatures far below the design wet bulb or dry bulb. It is not as dependant on the ambient temperature for the outlet coolant temperatures.
The weaknesses of a chiller are that it is a fairly complex piece of machinery. Chillers cost more than all other forms of cooling equipment. They require specialized periodic maintenance and trained certified repair technicians for proper operations. Chillers themselves introduce additional heat loading from the compressors that must also be removed in the condenser. The power required to operate a chiller is much higher than the other types of cooling systems discussed above. Cold weather operation of chillers requires special additional components on the chiller. Load variations may require special controls and/or multiple chiller circuits for efficient operation all adding to the overall cost of the equipment.
As you can see there are many types of cooling systems available to satisfy your requirements. It is best to involve your cooling system specialist early in your planning to help you choose the best system to fit your needs.
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