A seminar report on Opportunities for efficiency enhancements of water cooled chiller

Abstract

Air conditioning and chilling are very much in use not only in the context of industries but also domestically. Especially, water cooled chillers are used in places demanding considerably high cooling load. They are used in many manufacturing industries (like chemicals, pharmaceuticals, dairy, food, beverage etc.), agricultural & horticultural sectors (mainly cold stores) and commercial buildings (like hotels, hospitals, offices, airports, theatres, auditoria, multiplexes etc). The consumption of energy by the chiller is very significant. A huge proportion of energy is consumed by this plant in an industry. Its a practice and a trend in Nepal that chillers here are installed without much prior study and analysis. And at the same time, chillers are left to operate haphazardly without the knowledge of optimization. So if a proper analysis and study is made regarding the operation and installation of the chiller plant and the right procedure is followed then a huge amount of savings can be made. Efficiency enhancement of chiller plant not only provides economic benefit but also helps in assuaging the environmental degradation.

Introduction

Chilleras the name implicates, is equipment that removes heat from a liquid. And this liquid can be used to cool off air or products as required. This may work according to the principle of vapour compression system and absorption refrigeration system or the fusion of these two. However, Vapour compression machines, usually with electrically driven compressors, are the most commonly used machines for refrigeration and air conditioning for temperatures ranging from 25C to -70C [1]. In an air conditioning system the chiller water is generally distributed to the air handling units or other types of heat exchangers which cools the air or the required product. This chilled water is again re-circulated which is cooled again by the help of a refrigerant. In case of industrial application, they are often used in the plastic industry in injection and blow molding, metal working cutting oils, welding equipment,die-castingand machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling[1]. They are also used to cool high-heat specialized items such as MRI machines and lasers [1] and in hospitals, hotels and campuses.

Chillers are of two types:Air cooledWater cooled

Air cooled Vs Water cooled

A differentiating feature of the types of chillers is the method used to condense the refrigerant as it leaves the compressor. The two methods involve using either air-cooled or water-cooled condensers. Air-cooled condensers employ ambient air as the condensing medium and use a fan to move the air over the coil. Water-cooled condensers employ water as the condensing medium and use a pump to circulate the water through the condenser and out to a cooling tower that rejects the heat to the atmosphere. Operating cost is one of the primary factors when deciding between air-cooled or water-cooled chillers. Air-cooled chiller systems typically have a lower first and maintenance cost since they do not require a cooling tower, condenser water pumps, and associated condenser water chemical treatment. Operating costs, however, generally favor water-cooled chillers. This is be-cause water-cooled chillers can take advantage of lower condensing temperatures than air-cooled chillers.

Why water-cooled rather than air-cooled?

To take advantage of the wet bulb temperature, use of air-cooled condensers should be avoided for large cooling loads. Air cooled condensers may be permitted only for small cooling loads or in conditions of extreme scarcity of water or lack of space for cooling tower. Condenser water may be provided at the lowest acceptable temperature. The performance of air-cooled condensers is limited by the dry bulb temperature. The performance of these condensers can be improved, in dry weather conditions, by providing humidified air near wet bulb temperature. This pre-cooler consists of a cooling pad (with trickling water) through which the air is drawn. Depending on the design, a booster fan may be required to overcome the additional resistance to air flow. The potential for energy saving in dry summer months may be about 30% to 40% [2].

Air-cooled chillers have a full load kW/ton of approximately 1.25 while water-cooled chillers have a full load kW/ton of between 0.55 and 0.8 kW/ton [2]. The kW draw of the cooling tower fans and condenser water pump should be added to the water-cooled chiller kW/ton for an even comparison. Even after accounting for this added auxiliary energy draw, water-cooled chilled-water systems normally have an efficiency advantage over air-cooled.[2]So if a large ton of cooling capacity is required then water cooled chillers are installed due to its lower operating cost due to the fact that it has a higher efficiency. But for a relatively low ton of cooling capacity air cooled chillers are used due to its low initial cost which makes good trade off for its lower efficiency.

Refrigeration System Efficiency

The cooling effect of refrigeration systems is generally quantified in tons of refrigeration. The unit is derived from the cooling rate available per hour from 1 ton (1 short ton = 2000 pounds = 907.18 kg) of ice, when it melts over a period of 24 hours.British measuring units are still popularly used by refrigeration and air conditioning engineers; hence it is necessary to know the energy equivalents.1 Ton of Refrigeration (TR) = 3023 kcal/h= 3.51 kW thermal= 12000 Btu/hr

The commonly used figures of merit for comparison of refrigeration systems are:Coefficient of Performance (COP), Energy Efficiency Ratio (EER) and Specific PowerConsumption (kW/TR).

If both refrigeration effect and work done by the compressor (or the input power) are taken in the same units (TR or kcal/hr or kW or Btu/hr), the ratio isCOP = Refrigeration Effect Work done (Higher COP means better efficiency)

The other commonly used and easily understood figure of merit isSpecific Power Consumption = Power Consumption (kW) Refrigeration effect (TR)(Lower Specific Power Consumption implies better efficiency)

If the refrigeration effect is quantified in Btu/hr and work done is in Watts, the ratio isEER = Refrigeration Effect (Btu/hr) Work done (Watts)Higher COP or EER indicates better efficiency.

As mentioned earlier the chiller works according to principle of vapour compression refrigeration (VCR) or vapour absorption system (VAS). However, from the visit I made in some renowned companies in Nepal, I found that the chillers plants used there, employed chillers with vapour compressed refrigeration. The two systems differ in that the absorption cycle uses a heat operated generator to produce pressure differential where the mechanical compression cycle uses a compressor. The absorption cycle substitutes physic-chemical process for the purely mechanical process of the compression cycle.

Vapour compression system

Fig: vapour compression system [2]

Heat flows naturally from a hot to a colder body. In refrigeration system the opposite must occuri.e. heat flows from a cold to a hotter body. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or evaporates at a low pressure to form a gas. This gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air or water and turns back (condenses) into a liquid. In this way heat is absorbed, or removed, from a low temperature source and transferred to a higher temperature source.

Methods for the improvement of the COP of the refrigerant system

Under cooling (sub-cooling) of liquid refrigerant

In this method the refrigerant coming out of the condenser is further cooled more than its condensing temperature. For example, if the condensing temperature of the refrigerant is 450C but it is further cooled to 400C, then this called under cooling (sub-cooling) of the refrigerant. By the help of sub-cooling the refrigerating effect can be increased with the same input work by the compressor.

T2 Fig: Sub-cooling in vapour compression system [3] fig: T-S Diagram [3]

From the T-S diagram we see that,The initial COP = T1 T1-T2COP after sub-cooling, COP= T1 T1-T2As T2 > T2,COP > COP

This can be done by the help of different processes which are stated below:

By the use of under-cooler By the help of the refrigerant By the help of liquid refrigerant using low temperature liquid refrigerant

Compressing the refrigerant in two stages:

In this method the COP of the cycle is improved by reducing the net work done by the compressors keeping the refrigerating effect constant. This is done by compressing the gas coming out from the evaporator in two stages with inter cooling of the gas. This is generally used when the pressure ratio of compression is considerably high and low temperature refrigerating system. The effect of super heating of the refrigerant on the COP

Optimize process heat exchange

There is a tendency to apply high safety margins to operations, which influence the compressor suction pressure / evaporator set point. For instance, a process-cooling requirement of 150C would need chilled water at a lower temperature, but the range can vary from 60C to about 100C. At chilled water of 100C, the refrigerant side temperature has to be lower (about 50C to +5oC) [4]. The refrigerant temperature determines the corresponding suction pressure of the refrigerant, which in turn determines the inlet duty conditions for the refrigerant compressor. So the refrigerant temperature should be maintained to an optimum level, which means preventing overcooling. Minimizing energy consumption can be achieved in the following ways:

Proper sizing of heat transfer areas of process heat exchangers and evaporators

The heat transfer coefficient on the refrigerant side can range from 1400 2800 watts /m2K. The refrigerant side heat transfer areas are of the order of 0.5 m2/TR and above in evaporators [4].

Optimizing the driving force, i.e. the difference between evaporator temp Te and condensing temp Tc.

As we know that the theoretical formula for the COP is given by: TeTe-Tc

Therefore the difference between evaporator temp Te and condensing temp Tc should be kept at an optimum level.The approximate thumb rule is that for every 1C higher temperature in the evaporator, the specific power consumption will decrease by about 2 to 3% [1].

Table: Effects of Te and Tc on the cooling effect TR and the power consumption. [5]

Evaporator temperature(0C)Condenser temperatures(0C)

+35+40+45+50

+5Capacity(TR)151143135127

Power cons. (KW)94102.7110.6117.8

Sp. Power (KW/TR)0.620.720.820.93

0Capacity(TR)129118111104

Power cons.(kW)9096.8103108.9

Sp. Power (kW/TR)0.700.820.931.05

-5Capacity(TR)103969084

Power cons.(kW)84.289.694.799.4

Sp. Power (kW/TR)0.820.931.051.19

The conclusion is that you try to keep the difference between Te and Tc at an optimum level to ensure the best TR at the lowest power consumption.

Maintain heat exchanger surfaces

This is one of the most important factors that is required for the full fledged functioning and efficiency enhancement of all equipments and a chiller plant is no exception. Once compressors have been purchased, effective maintenance is the key to optimizing power consumption. Poor maintenance forces a compressor to work harder, which results in increased power consumption.

The following points should be kept in mind for the well-being and efficiency improvement of a compressor. Ensuring proper separation of the lubricating oil and the refrigerant Timely defrosting of coils Increasing the velocity of the secondary coolant (air, water, etc.).

Equally important is proper selection, sizing, and maintenance of cooling towers. A reduction of 0.550C in temperature of the water returning from the cooling tower reduces compressor power consumption by 3% . [4]

Table: Effect of poor maintenance on compressor power consumption. [5]ConditionTe (0C)Tc (0C)Refrigeration Capacity* (TR)Power Consumption (kW/TR)Increase kW/TR (%)

Normal7.240.517.00.69-

Dirty condenser7.246.115.60.8420.4

Dirty evaporator1.740.513.80.8218.3

Dirty condenser and evaporator1.746.112.70.9638.7

The above table shows the effect of poor maintenance practice upon the power consumption of the compressor. This can lead to increase of specific power consumption by more than 38%. So the proper maintenance of the equipments is very important.

Use Evaporators and Condensers with Higher Heat Transfer Efficacy

Use Heat Exchangers with Larger Surface Area

In the industries of Indian sub-continent, the specific power consumption for chilled water at 6 to 8C, in reasonably well maintained vapour compression systems, is likely to be around 0.8 kW/TR (only for compressor; pumps & fans are not included) [1]. The best systems available around our region today can give specific power consumption lower than 0.6 kW/TR (compressor power). In the USA, the specific power consumption figure of chillers is expected to be below 0.56 kW/TR to qualify as a highly efficient chiller [1]. A high efficiency chiller developed by Trane, USA, has a specific power consumption of 0.48 kW/TR [1]. This low specific power consumption has been achieved mainly by use of larger and more effective heat transfer area in the chillers and condensers. Larger area implies more effective heat transfer. This, in turn, implies that the refrigerant temperatures, for the same heat load, will be higher in the evaporator and lower in the condenser. Hence by using heat exchangers having large surface area can result in lower power consumption.

Use Plate Heat Exchangers for Process and Refrigeration Machine Condenser Cooling

The use of Plate Heat Exchangers for condenser cooling can lead to lower temperature approach, hence reducing the compressor energy consumption. Plate heat exchangers have a temperature approach of 1C to 5C instead of around 5C to 10C for shell and tube heat exchangers [4].

Matching capacity to system loadConsideration of part-load operation is important, because most refrigeration applications have varying loads. The load may vary due to variations in temperature and process cooling needs. During part-load operation, the evaporator temperature rises and the condenser temperature falls, effectively increasing the COP. But at the same time, deviation from the design operation point and the fact that mechanical losses form a greater proportion of the total power negate the effect of improved COP, resulting in lower part-load efficiency.Matching refrigeration capacity to the load is a difficult exercise, requiring knowledge of compressor performance, and variations in ambient conditions, and detailed knowledge of the cooling load.

Types and Capacity control of compressors

There are a variety of compressors available that can be used in the chiller plant. The commonly used compressors for most vapour compression systems are reciprocating, screw and centrifugal. So its very essential to choose the most appropriate compressor for the plant. The selection of the compressors mainly depends upon the cooling capacity, degree of vary of load, type of refrigerant, volume of refrigerant etc. If proper selection of the compressor can be made then it can help us to make large savings.

Table: Different facts associated with different types of compressors [6]

Compressors

ReciprocatingScrewCentrifugal

Available capacities0.5 to 150 TR70 to 750 TR90 to 1000 TR

COP at full load0.7-1 kW/ton1.1-1.5 kW/ton0.5-0.6 kW/ton

Refrigerants usedR11, R123, R134a, ammoniaR22, R134, ammoniaR22, R12

Advantages Low cost High COP at low capacity Simple and easy control Relatively higher compression ratio. Few moving parts. Compact, smaller and lighter. Quieter and low vibration. Designed for long periods of operation. Can tolerate liquid slugging. High efficiency at full load. As it is not constant displacement, so offers a wide range of capacities. Appropriate for high refrigerant volume at low pressure.

Disadvantages High level of maintenance due to more moving parts. Generates more noise and vibrations. Not appropriate for capacity more than 200 ton. High cost Not appropriate for small cooling load.

Has a very low COP at part load. At low part loads, prone to surging which can damage the compressor.

Recommendations regarding the use of compressors:Chillers use one of four types of compressor: reciprocating, scroll, screw, and centrifugal. The choice leans towards reciprocating compressors for peak loads up to 80 to 100 tons. Between 100 and 200 tons peak cooling load, two or more reciprocating compressor chillers can be used. Reciprocating compressors are of three types: open type, hermetic and semi-hermetic. Open type compressors are usually more efficient than the hermetic and semi-hermetic types because the suction vapour in a hermetic compressor passes over the motor to cool it, resulting in super heating of the vapour and thereby requiring more power for compression. However, proper refrigeration system design can minimize the impact on energy consumption. Above 200 tons, screw compressor systems begin to become cost effective. The screw chillers are well suited for applications demanding up to 750 TR. Above these capacities, centrifugal chillers are generally more cost effective where water is available for heat rejection. Centrifugal compressors traditionally provide larger capacities typically above 750 tons. The centrifugal machines offer highest peak load efficiency and operate reliably for applications demanding a steady state operation. The machines are only recommended with water-cooled condenser option.

Part Load Efficiency: In air conditioning systems, the peak load occurs only for a very limited number of hours during the year. On an annual basis, the imposed load will vary based on the time of day due to occupancy patterns, variation in product demand (such as in bevarage manufacturing industries) solar heat gains and the time of year due to solar and temperature seasonal variations. With all of these variables, every chilled water system operates at off-design conditions most of the time. Various studies indicate that a chiller is at 100% capacity about 1% of the time, 75% capacity about 42% of the time, 50% capacity about 45% of the time, and 25% capacity about 12% of the time [6]. System part load performance is thus a crucial factor in chiller selection. Simply put, the system part load, when multiplied by total annual ton-hours of cooling, provides an estimate of the total annual kilowatt- hour consumption or the chiller with the lowest system part load performance will provide the greatest energy savings across its entire operational range. Part load efficiency of various options is stated below: Screw chiller offer infinite reduction from 100% to 0% and affords good part load efficiency [6]. Centrifugal chiller accomplishes the capacity reduction in stages. When building load decreases, the chiller responds by partially closing its inlet vanes to restrict refrigerant flow. While this control method is effective down to about 20% of chillers rated output, it results in decreased operating efficiency [6]. For example a chiller rated at 0.6 kW per ton at full load might require as much as 0.9 kW per ton when lightly loaded [6]. Reciprocating compressors stepped capacity control is most efficient at minimum load than the twin-screw compressor. It is easy to closely match the capacity of the chiller to the building load by installing multiple machines which allow the facility manager to stage operation for part-load conditions, increasing operating efficiency. Chilled water storage

Depending on the nature of the load, it is economical to provide a chilled water storage facility with very good cold insulation. Also, the storage facility can be fully filled to meet the process requirements so that chillers need not be operated continuously. This system is usually economical if small variations in temperature are acceptable. This system has the added advantage of allowing the chillers to be operated at periods of low electricity demand to reduce peak demand charges. Low tariffs offered by some electric utilities for operation at nighttime can also be taken advantage of by using a storage facility. An added benefit is that lower ambient temperature at night lowers condenser temperature and thereby increases the COP. If temperature variations cannot be tolerated, it may not be economical to provide a storage facility since the secondary coolant would have to be stored at a temperature much lower than required to provide for heat gain. The additional cost of cooling to a lower temperature may offset the benefits.

Reduction in Heat Loads

Keep Unnecessary Heat Loads OutUnnecessary heat loads may be kept outside air-conditioned spaces. Often, laboratory ovens are kept in air-conditioned spaces. Such practices may be avoided. Provide dedicated external air supply and exhaust to kitchens, cleaning rooms, combustion equipment etc. to prevent easy mixing of air between warm and cool rooms due to pressure differentials. In cold stores, idle operation of fork lift trucks should be avoided in case of any unforeseen stoppage of material movement.

Use False CeilingsAir-conditioning of unnecessary space wastes energy. In rooms with very high ceiling, provision of false ceiling with return air ducts can reduce the air-conditioning load.

Use Pre-Fabricated, Modular Cold Storage Units

Cold stores should be designed with collapsible insulated partitions so that the space can be expanded or contracted as per the stored product volumes. The idea is to match product volumes and avoid unnecessary cooling of space and reduce losses. Modular cold store designs are commercially available.

Minimizing Heat Ingress

Check and Maintain Thermal Insulation

Repair damaged insulation after regular checks. Insulate any hot or cold surfaces. Replace wet insulation. Insulate HVAC ducts running outside and through unoccupied spaces. Provide under-deck insulation on the ceiling of the top most floor of air conditioned buildings.

Insulate Pipe Fittings

Generally, chilled water/brine tanks, pipe lines and end-use equipment in the industry are well insulated. However, valves, flanges etc. are often left uninsulated. With rising energy costs, it pays to insulate pipe flanges, valves, chilled water and brine pumps. For outdoor piping, provide metal cladding for weather protection.

Use Landscaping to the Reduce Solar Heat Load

At the time of design of the building, fountains and water flow can be used to provide evaporative cooling and act as heat sinks. Trees may be grown around buildings to reduce the heat ingress through windows and also reduce glare. Terrace lawns can help reduce the solar heat gain.

Reduce Excessive Use of Glass on BuildingsModern commercial buildings use glass facades or large window area resulting in large solar heat gain and heat transmission. Such architecture is suitable for cold countries; but in countries like ours, it increases the air conditioning load for about eight to ten months in a year [1]. In existing buildings, the possibility of replacing glass panes with laminated insulation boards should be seriously considered. The colour of the laminations can be chosen to suit the internal and external decor.

Use Glass with Low Solar Heat Gain Coefficient and Thermal Conductivity

The table below shows the solar heat gain coefficient (SHGC), thermal Conductivity and daylight transmittance for different types of glass panes. Use of glass with low SHGC and thermal conductivity is recommended. Daylight transmittance is important, if electric lighting (another heat load on air conditioning) has to be minimized.

Table: Solar heat gain coefficient (SHGC), thermal Conductivity and daylight transmittance for different types of glass panes [1].

Product Solar Heat Gain Coefficient (SHGC)Thermal ConductivityDaylight Transmittance

Clear Glass0.723.1679

Body Tinted Glass0.453.2465

Hard Coated Solar Control Glass 0.263.2724

Soft Coated Solar Control Glass0.183.0815

Low Emissivity Glass0.562.3361

Solar Control + Low Emissivity Glass0.231.7741

Use Low Conductivity Window Frames

We should consider the use of plastic window frames in place of Steel and Aluminium frames. This can reduce the heat ingress by conduction. Provide Insulation on Sun-Facing Roofs. Air-conditioned hotels and corporate office buildings should be constructed with insulated walls to reduce the heat ingress. Providing under roof insulation is also an economical way of keep the house thermally resistant. Add vestibules or revolving door or self-closing doors to primary exterior doors. Air curtains and/or PVC strip curtains are recommended for air-conditioned spaces with heavy traffic of people or pallet trucks. Use intermediate doors in stairways and vertical passages to minimise building stack effect.

Optimization of cooling towers

A cooling tower is an integral part of a chiller plant. This is used for cooling of the condenser and the compressor. Pumps and fans are the devices used in the cooling tower that uses up the electrical energy. A huge savings can also be made if the size of the cooling tower can be reduced. If proper optimization and selection of the cooling tower is done then large savings can be obtained. There are different parameters used assessing the parameters of a cooling tower. They are as follows:

1. Range(Difference between cooling water inlet and outlet temperature)2. Approach(Difference between cooling tower outlet cold water temperature and ambient wet bulb temperature)3. Effectiveness(Range / (Range + Approach))4. Cooling capacity(Heat rejected in kCal/hr or tons of refrigeration (TR)5. Evaporation loss(Water quantity (m3/hr) evaporated for cooling duty)6. Cycles of concentration(Ratio of dissolved solids in circulating water to the dissolved solids in make-up water)7. Blow down losses(Evaporation Loss / (C.O.C. 1))8. Liquid / Gas ratio(Ratio between water and air mass flow rates)

The main areas for improving the energy efficiency of cooling towers are:

Selecting the right cooling tower (because the structural aspects of the cooling tower cannot be changed after it is installed)

Once a cooling tower is in place it is very difficult to significantly improve its energy performance. A number of factors are of influence on the cooling towers performance and should be considered when choosing a cooling tower: capacity, range, approach, heat load, wet bulb temperature, and the relationship between these factors.Capacity, in terms of heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are an indication of the capacity of cooling towers. However, these design parameters are not sufficient to understand the cooling tower performance. For example, a cooling tower sized to cool 4540 m3/hr through a 13.90C range might be larger than a cooling tower to cool 4540 m3/hr through 19.50C range [7]. Therefore other design parameters are also needed. Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9oC to 32.2oC at 26.7oC wet bulb temperature [7]. When the size of the tower has to be chosen, then the approach is most important, closely followed by the flow rate, and the range and wet bulb would be of lesser importance.As a general rule, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size [7]. Usually a 2.80C approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee [7].

Heat Load

The degree of cooling required is controlled by the desired operating temperature of the process. The size and cost of the cooling tower is increases with increasing heat load. Purchasing undersized equipment (if the calculated heat load is too low) and oversized equipment (if the calculated heat load is too high) is something to be aware of.

Wet bulb temperature considerations:

Wet bulb temperature is an important factor in performance of evaporative water cooling equipment, because it is the lowest temperature to which water can be cooled. For this reason, the wet bulb temperature of the air entering the cooling tower determines the minimum operating temperature level throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature. In practice, however, water is cooled to a temperature higher than the wet bulb temperature.A pre-selection of towers based on the design wet bulb temperature must consider conditions at the tower site. In general, the design temperature selected is close to the average maximum wet bulb temperature in summer. It is better to conform the wet bulb temperature with the ambient temperature than the inlet air temperature as this can be affected by discharge vapour.The cold-water temperature must be low enough to exchange heat or to condense vapors at the optimum temperature level. The quantity and temperature of heat exchanged can be considered when choosing the right size cooling tower and heat exchangers at the lowest costs.

Relationships between range, flow and heat load

The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range: The inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is economical to invest in removing the additional heat. The exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.

Relationship Approach and Wet bulb temperature

The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a 16.670C range and a 4.450C approach to 21.110C wet bulb would be larger than the same tower to a 26.670C wet bulb [7]. The reason is that air at the higher wet bulb temperature is capable of picking up more heat. This is explained for the two different wet bulb temperatures: Each kg of air entering the tower at a wet bulb temperature of 21.10C contains 18.86 kCal. If the air leaves the tower at 32.20C wet bulb temperature, each kg of air contains 24.17 kCal. At an increase of 11.10C, the air picks up 5.31 kCal per kg of air [7]. Each kg of air entering the tower at a wet bulb temperature of 26.670C contains 24.17 kCals. If the air leaves at 37.80C wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.10C, the air picks up 15.5 kCal per kg of air, which is much more than the first scenario [7].

Fills

In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways: Electricity is used for pumping above the fill and for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption. Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes.

Table: Comparison of splash fill and film fill [7].

Splash Fill Film Fill

Possible L/G Ratio 1.1 1.5 1.5 2.0

Effective Heat Exchange Area 30 45 m2/m3 150 m2/m3

Fill Height Required 5 10 m 1.2 1.5 m

Pumping Head Requirement 9 12 m 5 8 m

Quantity of Air Required High Much Low

We see that the fill film is more efficient than the splash fill, as the heat transfer is more effective here. However, the splash fill is more supportive for the dirty water and environment and is easier for maintenance.

Similarly the cooling tower efficiency can be enhanced further by optimizing pumps, water distribution system, fans and motors.

Conclusion and recommendations

With industrial development and augmentation of the life style of the people, the demand for refrigeration and air conditioning is proliferating. Chiller plants are an integral part of many industries. As mentioned earlier it is a plant utilizing a large proportion of energy. If proper optimization of chiller plant is made then the efficiency of the chiller plants can be enhanced which can help to minimize the energy consumption, which will ultimately contribute towards cost savings as well as reducing the environmental effects. However in Nepal no such practices and procedures are followed for the efficiency enhancement. Even during the installation of the chiller plant, proper scientific heat load calculations are not done. On many cases, approximation simply with reference to the room size is made. Many factors such as air infiltration, solar heat load and others are not taken into account during the heat load calculation. Similarly, the operators of the chiller plant (esp, in industries) arent well trained, for the optimum chiller operating procedure. Every technical work force involved with the chiller plant (this includes operators and engineers) should be well trained about how they could enhance the efficiency of the chiller plant. As with the chiller applications such as in different commercial buildings, people should use it in a wise and thrifty way. Similarly periodic maintenance of the plant should be highly encouraged.However the most essential point is that everybody should prioritize towards the use of nature-assisted cooling techniques and minimal use of energy guzzling refrigeration equipments is the key energy conservation.

References

[1] Best practice manual on HVAC chillers, 2006 (Devki Energy Consultancy, India)[2]Commercial HVAC chiller equipment (Technical Development Programme, USA, hvacpartners.com)[3]A course in Refrigiration and Air-conditioning, 8th edition (By Arora, Domkundwar)[4]Energy efficiency guide on Air-conditioning and Refrigiration (UNEP, 2006)[5]ASHRAE Handbook[6]Overviews of chiller compressors (A. Bhatia)[7]Energy efficiency guide on Cooling towers (UNEP, 2006)