EVAPORATIVE COOLING APPLICATIONS

EVAPORATIVE COOLING APPLICATIONS

Direct Free Cooling Applications

The condenser of the cooling unit is cooled by water in systems using cooling towers. The condenser is of the shell and tube type. The coolant vapour passing outside the pipes is cooled and condensed by the condenser water circulating within the pipes.  In this stage, the cooling of the condenser water is done by the tower. This is how the condenser water is cooled when the chiller is operated in high outdoor temperatures. However, as the ambient temperature falls below the temperature of the cold water used in the cooling system, the cooling group no longer needs to be used. In this case, in Direct Systems, the chiller is bypassed and the condenser water from the tower is sent to the system directly as the cooling water required by the system. The basic advantage of this system in towers operating as open circuits can be summarised as follows: The required cold water temperature can be approximated to the wet bulb temperature of the environment. In this case, the maximum benefit will be gained from Free Cooling. However, a serious disadvantage of the system is the pollution caused by the relatively dirty condenser water in the clean cold water system. Even though attempts to alleviate pollution by methods like filtration, this system has been lately unpopular among applicators. Cooling tower applications operating as closed circuit-indirect systems or dry cooler applications catering to the same demands eliminate this problem.

Indirect Free Cooling Applications

In Indirect Systems, the condenser water circuit and cold water circuit are separate. The pollution risk which is an issue in open circuit systems does not exist in indirect-closed circuit applications. However, annexing an additional heat transfer surface to the system leads to a higher water temperature as compared to open circuit water cooling towers. This cause the efficiency of Free Cooling to somewhat drop.

Indirect systems can be applied in three main ways:

1.Systems utilizing closed circuit cooling towers:

The use of Closed Circuit Cooling Towers is common in Free Cooling. As mentioned above, it eliminates the risks of pollution, etc. seen in open circuit systems. In the summer season which is the normal operating period of the cooling group, the condenser water coming from the tower circulates from the condenser within a closed circuit. In winter months however, when only Free Cooling is used, the water coming from the tower circulates in the cold water circuit within the closed circuit.

Figure 2.b shows the Load Sharing Closed Circuit Cooling Towers Free Cooling application. In this system, the returned cooling water undergoes a pre-cooling process prior to entering the evaporator. This application decreases the cooling load on the chiller. This application increases system efficiency in interim seasons.

In water cooling towers, depending on design, water can be cooled to a temperature up to 3-6ºC above wet bulb temperature. In closed circuit cooling towers, the water temperature which can be attained is up to above 2-3ºC the temperature that can be achieved with open circuit water towers. In ideal conditions wet bulb temperature can be approached closely by increasing tower dimensions; however in this case the cost of investment will increase dramatically. While water cooling towers are advantageous in terms of initial investment cost, they can lead to problems in operation. In case the water from the water tower is used directly, this causes calcification and pollution of cooling exchangers. Furthermore, the portion of the water used in water towers which is lost to evaporation needs to be constantly replenished. Additionally, the water needs to be flushed periodically in order to get rid of sedimentation within the tower tank; the water in the tank may reach extremely high degrees of hardness and pollution dues to the accumulation of scale and other similar substances in water diminished by evaporation in the system. The use of these systems should also be evaluated in terms of water loss and water quality.

2.Systems Utilizing Supplemental Heat Exchangers:

These are applications utilizing a separate heat exchanger within the system. The heat exchangers used in these cases are usually plate type ones. In the winter season, the heat load in the cold water circuit is received by the tower water through a supplemental heat exchanger without the need for a chiller.

3.Refrigerant Gas circulation system:

The Refrigerant Gas circulation system is one which is rarely used. In cases where the condenser water can be obtained below the desired cold water temperature, the chiller operates like a thermosyphon. The low temperature condenser water condenses the gaseous refrigerant fluid within the condenser. The condensed refrigerant fluid is transferred to the evaporator by the aid of gravity or an auxiliary pump. The high temperature cooling water circulating in the system causes the refrigerant fluid within the evaporator to vaporise. The differential pressure between the Evaporator and the Condenser causes the gas to return to the Condenser. In this system, the flow between the evaporator and the condenser is facilitated through by-pass connections. This system renders the operation of a compressor unnecessary. This system is not applicable to all cooling groups. And in groups where it is applied, the Free Cooling capacity is limited to 10-30% of the design capacity of the chiller. The Free Cooling capacity depends on the design specifications of the chiller and the temperature difference between the desired water temperature and the condenser water temperature.

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