Adiabatic Cooling Tower Photo Credit: https://almeco.eu/en

Chiller provides the required conditioned air for various applications, ranging from offices, schools, malls, commercial buildings etc. Modern workspaces, in high rise, glass enclosed buildings would not have been possible without the availability of the provisions of air-conditioned air. While there are a number of naturally ventilated buildings which do not require mechanical systems, the majority of work spaces need some form of air conditioning. The air conditioning industry has undergone transformational changes in the last three – four decades, with advances in every area of the HVAC environment. Component as well as system level efficiencies have been improving steadily and new technologies have helped the occupiers and owners of the assets reduce operating costs and improve equipment life.

With a rapidly changing market environment and new technologies coming in by the day, building owners and maintenance teams are always on the lookout on ways to keep pace with the changes so that their HVAC systems and equipment are working at higher efficiencies and lower costs. Retrofits of existing systems enable the asset owners to incorporate latest technologies and improve system performance. Air cooled chillers are gaining traction in the commercial and retail sectors due to the lower maintenance costs and faster set up times. A key disadvantage that air cooled chillers have is their relatively higher overall energy consumptions. Adiabatic cooling systems retrofitted on existing air
Photo cooled chillers are a way of improving the performance of the plant using a simple technology with limited changes and at a low initial cost. Just as any product has advantages, there are of course some disadvantages of this technology so the implementation has to be based on the requirement for that application.

Overview of Adiabatic Cooling

Air cooled chillers have many advantages over water cooled chillers – lower maintenance costs as the number of components is less, cheaper to maintain, reduced complicity views no separate cooling towers, pumping systems etc. Air cooled chillers are not effective when the ambient temperature becomes very high, especially, in the critical peak temperature days that are experienced in the northern belt of India. The main attraction for water cooled chillers is the lower energy consumption as compared to air cooled chillers. The difference in energy can vary from 10 – 25% depending on the application, location and type of product. Air cooled chillers of course have the main advantage of using very less water which makes them beneficial in regions where water availability is limited.

A typical air-cooled chiller working principle is shown in figure 1. Refrigerant gas absorbs the heat form the chilled water and is compressed as in a standard chiller operation. The condensation occurs in the cooling coils rather than in a condenser and the heat exchange medium is air passed across the cooling coils at high speed. Thus, the cooler the air passing across the coils, the better will be the condensation and lower will be the energy utilization.

Figure 1: Air Cooled Chiller set up

Adiabatic cooling technology or “precooling” for air cooled chillers essentially reduces the temperature of the air passing across the cooling coils by spraying water into the incoming air stream. Figure 2 shows the arrangement of the adiabatic cooling system retrofitted onto an aircooled chiller.

Figure 2: Pre Cooling System on an Air Cooled Chiller

How Adiabatic Cooling Works

The principle is very simple – water is sprayed in the form of a mist onto the incoming air stream. The water increases the relative humidity of the air, causing heat to transfer from the hot, ambient air to transfer of heat by way of evaporation. This reduces the temperature of the air moving across the coils.

Special screens are fitted in front of the condenser coils which are designed to minimize the pressure drop of the incoming air stream. The screens are non-metallic to avoid ill effects of water. Spray jets are positioned in front of the screens which spray a measured amount of water in the form of a mist onto the screens. The control for the spray jets is typically digital, with quantity and frequency being the variables that are controlled by the system.

When the ambient temperature exceeds set values, the spray system engages and water is sprayed onto the screens. The air stream temperature is monitored on a continuous basis and is used as a feedback for the jet spray flow and timing.

Treated soft water is used to avoid clogging of the sensitive jet nozzles. In addition, water is also UV treated to avoid any biological growth and prevent possibility of legionella disease. While most of the water evaporates, there is some amount of the spray water that drips down the screens and is collected in basins. This water can then be reused after treatment or drained out.

Benefits and Drawbacks of Adiabatic Cooling

In principle, an adiabatic pre-cooling system fitted air cooled chiller or the system installed as a retrofit appears to be a win proposition for the chiller owners. However, as is the case for any system, there are benefits that can be accrued through this technology and there are also some drawbacks that limit the use of this approach.

Benefits: Factory fitted pre-cooling systems are not very common at present and most of the installations are retrofits. The advantages of retrofitting this system to an existing air-cooled chiller are as below:

• The key advantage that this system brings is the reduction in energy costs due to the lowering of the discharge pressures. Studies have shown that a 10-degree reduction in air temperature can lower the power consumption by 12 – 14%
• Frequent plat tripping when the ambient temperature peaks can be avoided leading to higher system reliability and availability
• The screen help protects the cooling coils from direct sunlight and atmospheric
• Opportunity to use same system for enhanced cooling requirement due to the increased operating range. This enables savings on sizing of the system.

Some of the drawbacks of the precooling system which prevent the technology to be used in all installations are

• Requirement of treated water is the most critical aspect as the nozzles on the jet have very fine tolerances. Thus, water with the right quality of hardness is essential which requires a water treatment system to be installed
• To prevent legionella disease, the water requires to be treated. This is usually done by UV systems and again increases the complexity of the system.
• The cost of maintenance increases due to the added systems and pumping components. This negates the lower maintained effort of air cooled chillers
• Water usage while low, brings in added complicity into a traditionally less complex system as compared to a water-cooled chiller system
• The control systems are complex and hence will increase the cost as well maintenance efforts.

Thus, the choice to go for a precooling system depends on a various factor not only the initial cost benefits and each application requires a deeper study to arrive at the best possible option.

Case Study

To demonstrate the effectiveness of the pre-cooling system, an air-cooled chiller was retrofitted and data analyzed pre and post the installation as well as with similar units in the same building. The details of the set up are as below

• The retrofit was on a 330 TR air cooled chiller which was part of a bank of 8 chillers for a commercial building in the northern, dry climate zone, with peak summer temperatures above 45 degrees centigrade for a few days expected.
• RO water in use from the existing domestic water system and hence no additional costs were incurred for the water treatment
• Ph. inhibitors were used to prevent corrosion of the coils
• No separate UV treatment system was installed for this application
• The system has 34 nozzles and water spray is at a 30 sec frequency
• The approx. water usage is 75 liters per hour

Based on six months of running, the retrofitted system was seen to show an overall savings of 9% in energy usage as compared to the chillers without precooling systems. The cost of enhanced maintenance as well as water costs were not factored in arriving at the savings so the net lowering of cost would be lower than the absolute energy cost reduction.

Conclusion

Chillers consume the maximum amount of energy in a commercial building set up and hence, any opportunity to lower energy usage is an opportunity to lower operating costs for the owners and users. While good O&M practices and intelligent use of the chillers can help reduce energy usage, new technologies that can be retrofitted into the system will lead to large savings. The pre-cooling or adiabatic cooling system is an emerging retrofit option that can with low cost inputs, lead to sizable savings. The system is easy to install, does not need major modifications and the maintenance costs are low. There are a few drawbacks of the system but proper application of the technology in the right set up will enable the chiller O&M team to lower operating costs as well as save on energy.