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Closed cooling water systems
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closed systems

Unlike the once-through system discussed in the previous section, the makeup water usage in a closed system is usually minimal. Specifically, a system is considered closed if it does not employ open evaporation for cooling and has an average water loss of less than 0.5% of the circulation rate. Since the dissolved solids in the water are not concentrated by evaporation, deposition by scaling is usually not a problem.

Fouling is also minimized because the system is not open to outside contaminants. The main exception to this is microbiological fouling which can cause serious problems in closed systems. Be aware that these assumptions are based on ideal design conditions where the only water losses are minor ones due to pump seal leaks, expansion tank overflows and surface evaporation from system vents.

With those systems that have severe water losses and, consequently, high makeup rates, the potential for scale can become significant. Since there is no bleedoff in a closed system, there is an opportunity for rust or suspended solids to drop out in low flow areas and deposit on heat transfer surfaces.

Consequently, for those systems with high makeup rates, it is usually advisable to either use high quality makeup water (either softened or demineralized) or employ a corrosion inhibitor that is blended with deposit control agents. Later on, we will discuss methods by which to control the primary water problem associated with closed loop systems: corrosion.

 


Figure I Typical closed cooling system.

In engineering terms, the "closed system" is actually two interrelated systems: a completely sealed system for recirculation water and a chiller or heat exchanger used to cool or remove heat from it. The engine cooling system of an automobile provides a classic example of a closed recirculating system.

The engine gives off its generated heat to the water recirculated through it, and the water is cooled as it circulates through the radiator. In order to further clarify the concept of closed loop cooling, figure I-b shows a diagram of a simple closed recirculating system.

Basically, heat is transferred to the closed loop by typical heat exchange equipment and is removed from the closed system by an exchange of heat from the closed loop to a secondary cooling water cycle.

The secondary loop could use either evaporative or once-through water cooling, or air cooling. You should note, that, included in the diagram is an expansion tank (surge tank), which not only allows for temperature induced water volume changes but also functions as an addition point for makeup water. Basically, there are two types of expansion tanks: open and closed. Open tanks are vented to the atmosphere and are usually located at the highest point in the system.


Closed tanks, which operate under pressure, are not capable of venting oxygen and other dissolved gases that enter through the makeup water and leaking valves. Therefore, air vents must be installed at high points throughout the system. While these vents are designed to remove dissolved gases from the system, they can serve as an oxygen contamination point if changes in system pressure cause air to be drawn in through them. Leaking air vents can significantly accelerate corrosion in closed systems, especially those that operate at higher temperatures because oxygen is more aggressive at higher temperatures.

 

 

Curve A, plots data for a completely closed system without any means of removing dissolved oxygen. Curve B, plots data for a vented system. For reasons that we will discuss in more detail later, the rate of corrosion tends to increase with temperature. On curve A, there is a steady increase in corrosion as the temperature increases.

Curve B, on the other hand, shows a steady increase in corrosion up to approximately 170eF. At this temperature, the loss of dissolved oxygen through venting exceeds the amount made available through diffusion and, consequently, a decrease in the corrosion rate occurs.


 

 

Air Conditioning Systems

Generally speaking, all air conditioning equipment consists of two separate systems. A closed chill water system, which circulates throughout the building to provide the temperature reduction, and an open recirculation condenser water system, which serves to condense the refrigerant in use. Air conditioning capacity is measured in tons of refrigeration. A ton of refrigeration is defined as the capacity to remove heat at a rate of 12,000 BTU per hour at the evaporator or chiller. Most commercial or institutional cooling water systems fall under these major categories:

1. Reciprocating compressor units, usually electrically driven and using a fluorocarbon as a refrigerant.

2. Centrifugal compressor units, driven by electric motor, steam turbine or gas engine, also use a fluorocarbon refrigerant.

3. Absorption refrigeration units, in which the refrigerant (water) is absorbed in concentrated lithium bromide or chloride solution, and then evaporated by steam.

4. Free cooling, which adapts any of the previous types of equipment to directly inject chill water into the open recirculation cooling water system and eliminates the need for compressor or absorber operation.

Reciprocating or Centrifugal Compressor Units

The water handling equipment of both these units is identical and, therefore, can be handled as one type of system. A diagram of the main components of compression cycle air conditioning system is included. The "chiller" is a shell and tube heat exchanger with water on one side and fluorocarbon refrigerant on the other. Heat is transferred from the water to the refrigerant, causing the fluorocarbon to boil, and producing water chilled to approximately 40F. This water is pumped throughout the area to be cooled in a closed system.

The chilled water eventually reaches capillary piping within air handling coils; air flowing over these chilled water coils is cooled and dehumidified. The water, having lowered the air temperature, is returned to the chiller for temperature reduction and subsequent reuse.

 


The system is made continuous by compressing vaporized refrigerant, sending it to the condenser to be reliquified and then storing it in a receiver until it is released through an expansion valve for repeated duty to the chiller.

The "condenser" is another shell and tube heat exchanger which employs cooling tower water on the tube side to cool and liquify the heated refrigerant. Compression refrigeration systems are generally designed for a 10F temperature drop across the tower. For proper heat rejection, these systems require a cooling water recirculation rate of approximately 3 gpm per ton of refrigeration.

The major energy consumption in a compression refrigeration system lies in the operation of the compressor, which is designed to operate at a certain head pressure for a given load. This pressure is the refrigerant pressure in the condenser. The term, "high head pressure", means that the condenser head pressure is higher than it should be at a specific load condition. This is normally caused by some interference of heat transfer between the condenser water and the refrigerant. From a water treatment standpoint, the presence of scale or some other heat insulating deposit could cause this condition to occur.

High head pressure conditions increase the power consumption of the equipment because the compressor will offset the resistance to heat transfer by increasing the refrigerant pressure and the associated temperature. This increases the differential temperature between the refrigerant and the cooling water, thereby, increasing the driving force to transfer heat. If high head pressure continues to increase, the unit may shut down when the preset maximum head pressure is reached in the compressor.

 

Absorption Refrigeration Units

This design is popular for smaller air conditioning systems and is more complicated in its operation. Steam absorption units are hermetically sealed vessels, which use water as a refrigerant, a lithium bromide solution as refrigerant absorber and steam as a heat source to evaporate the water. A diagram of typical steam absorption unit is shown in Figure below.


As water flows from the condenser to the evaporator, a shell and tube heat exchanger, it evaporates and cools the chilled water for circulation in a closed cooling system comparable to that of a compression cycle system. The absorber, another shell and tube heat exchanger with the chilled water on the tube side is maintained at a lower pressure than the evaporator. The resulting differential causes the evaporated water refrigerant to flow to the absorber. There it comes in contact with an absorbing salt solution, lithium bromide.

This dilute refrigerant-salt solution then flows to the concentrator. At this point, the water is distilled from the lithium bromide solution, and enters the condenser for recycling; the concentrated lithium bromide flows back to the absorber for repeated duty.

In order to provide the heat needed for distillation in the concentrator, steam or high temperature hot water is required. Therefore, the name "steam absorption unit". With a 15F temperature drop across the tower, the heat rejection for an absorption system requires the circulation of approximately 4 gpm per ton of air conditioning.

Fouling or the formation of scale in an absorption system will also reduce operation efficiency. Since the highest water temperatures exist in the condenser, deposition will first occur in this unit. Under extreme conditions, scale formation can form in the absorber.

Deposition in the condenser causes a condition similar to high head pressure in a compression system. It creates a higher backpressure on the concentrator. In turn, increased steam or hot water is required to liberate the refrigerant from the brine solution.

This raises the refrigerant vapor temperature and the temperature differential between the water vapor and the cooling water. While this compensates for the resistance to heat flow, more energy is required to provide this increase in heat input.

If conditions are severe enough to cause deposition in the absorber, then capacity will be reduced. This is due to the absorption rate being reduced while the brine is being heated more than normally in the concentrator. This presents the potential to overconcentrate the brine solution, resulting in a "freeze-up" which will shut down the system.


 

 

Free Cooling

This method of air conditioning is a relatively new approach, which attempts to minimize energy consumption during off-peak cooling seasons. Normally, the existing air conditioning equipment is altered to include a fine mesh strainer in the water flow. With this in place, the closed chill water is mixed with the open recirculating condenser system and temperature reduction is achieved exclusively by evaporation over the cooling tower (see Figure I-f). This eliminates the expensive operation of the compressor and absorption equipment previously explained. Naturally, this mode of operation is only possible when the wet bulb temperature of the outside air is sufficiently low enough to provide water which is capable of handling the heat load of the space or building being serviced.

The strainer is included in the system to filter any airborne debris or filamentous microbes which could enter the chill water system and block the small i.D. openings of the cooling coils. The strainer is a conventional Adams type, with a battery of long cylindrical tubes which will self clean by automatically reversing flow when the pressure exceeds a preset differential. Normally, the backwash water is pumped to a sewer,

These systems are designed to be operated on a continuous or intermittent basis. Therefore, many times a given air conditioning plant may switch from conventional operation of separate condenser chill water systems, to the strainer-cycle which combines them into one system, and then back to the original operation when the outside temperature increases sufficiently to prohibit strainer-cycle operation.

 

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