Industrial Cooling
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This paper introduces the subject of industrial cooling and discusses the most important energy savings that are possible in this area. ...

This paper introduces the subject of industrial cooling and discusses the most important energy savings that are possible in this area.

Cooling is very expensive, so it is important that it is used only where necessary, and that only the most efficient technology is used. For thermodynamic reasons, the energy efficiency of a cooling system increases with decreasing temperature differential. It is therefore crucial to keep this differential as low as possible.

Three main types of cooling systems prevail in industrial environments: dry cooling, evaporative cooling, and compression cooling. This paper explains their main working principles and characteristics. Other types, such as absorption cooling, gas expansion, and thermo-electric cooling, are not treated in this application guide because of their limited presence in industry.

Each system has its own application domain. The choice of the right cooling system is one of the important initial decisions that must be taken in order to achieve maximum energy efficiency. Furthermore, this paper discusses several specific energy saving actions for each of the three cooling systems.

Significant energy savings can be made by installing variable frequency drives on fans (dry cooling, evaporative cooling), pumps (evaporative cooling, compression cooling), and compressors (compression cooling).

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Industrial Cooling Industrial Cooling Document Transcript

  • European Copper Institute APPLICATION NOTE INDUSTRIAL COOLING Nico Vanden Broeck, Laborelec October 2011 ECI Publication No Cu0117 Available from www.leonardo-energy.org /node/2020
  • Publication No Cu0117 Issue Date: October 2011 Page i Document Issue Control Sheet Document Title: Application Note – Industrial Cooling Publication No: Cu0117 Issue: 02 Release: October 2011 Author(s): Nico Vanden Broeck, Laborelec Reviewer(s): David Chapman Document History Issue Date Purpose 1 June 2007 Initial publication 2 October 2011 Upgrade to be adopted into the Good Practice Guide 3 Disclaimer While this publication has been prepared with care, European Copper Institute and other contributors provide no warranty with regards to the content and shall not be liable for any direct, incidental or consequential damages that may result from the use of the information or the data contained. Copyright© European Copper Institute. Reproduction is authorised providing the material is unabridged and the source is acknowledged.
  • Publication No Cu0117 Issue Date: October 2011 Page ii CONTENTS Summary ........................................................................................................................................................ 1 Introduction.................................................................................................................................................... 2 Dry Cooling ..................................................................................................................................................... 3 Advantages and disadvantages ..............................................................................................................................3 Energy Saving Possibilities on dry cooling systems ................................................................................................3 Evaporative cooling ........................................................................................................................................ 4 Advantages and disadvantages ..............................................................................................................................4 Cooling tower types................................................................................................................................................4 Open cooling tower ................................................................................................................................................4 Evaporative condenser and closed cooling tower..................................................................................................5 Hybrid cooling tower ..............................................................................................................................................6 Energy saving possibilities in the evaporative cooling domain ..............................................................................6 Why is a variable frequency drive so interesting? ...................................................................................6 Other aspects influencing the energy efficiency ......................................................................................7 Compression cooling....................................................................................................................................... 8 Theoretical and actual Carnot cycle .......................................................................................................................8 The Condenser .........................................................................................................................................9 The Expansion Valve.................................................................................................................................9 Evaporation Systems..............................................................................................................................10 Multiple compressor arrangement.......................................................................................................................10 Efficiency—COP ....................................................................................................................................................11 Ammonia versus other refrigerants .....................................................................................................................12 Energy saving possibilities on compression cooling .............................................................................................13 Conclusions................................................................................................................................................... 15 References.................................................................................................................................................... 15
  • Publication No Cu0117 Issue Date: October 2011 Page 1 SUMMARY This paper introduces the subject of industrial cooling and discusses the most important energy savings that are possible in this area. Cooling is very expensive, so it is important that it is used only where necessary, and that only the most efficient technology is used. For thermodynamic reasons, the energy efficiency of a cooling system increases with decreasing temperature differential. It is therefore crucial to keep this differential as low as possible. Three main types of cooling systems prevail in industrial environments: dry cooling, evaporative cooling, and compression cooling. This paper explains their main working principles and characteristics. Other types, such as absorption cooling, gas expansion, and thermo-electric cooling, are not treated in this application guide because of their limited presence in industry. Each system has its own application domain. The choice of the right cooling system is one of the important initial decisions that must be taken in order to achieve maximum energy efficiency. Furthermore, this paper discusses several specific energy saving actions for each of the three cooling systems. Significant energy savings can be made by installing variable frequency drives on fans (dry cooling, evaporative cooling), pumps (evaporative cooling, compression cooling), and compressors (compression cooling).
  • Publication No Cu0117 Issue Date: October 2011 Page 2 INTRODUCTION Cooling is, in general, an expensive form of energy. Industrial cooling typically consumes up to 7% of the national electrical consumption in Western Europe. The following rules of thumb are the basis for any industrial cooling concept:  The use of cooling should be reduced as much as possible  The most efficient technology must be used  The required temperature differential should be kept as low as possible Three main types of cooling plant satisfy 90% of the industrial market: dry cooling, evaporative cooling, and compression cooling (chiller). The useful temperature ranges of the three main types of cooling are illustrated in Figure 1. Figure 1: Main types of cooling and their usual operating temperature ranges. 40 35 25 20 T (°C) EVAPORATIVE COOLING (open, closed, hybrid,…) DRY COOLING COMPRESSION COOLING (CHILLER) (aircooled, watercooled)
  • Publication No Cu0117 Issue Date: October 2011 Page 3 DRY COOLING In dry cooling, fans drive ambient air over a warmer process fluid or gas (e.g. a glycol water solution) to cool it. This type of cooling is used when the required low temperature is above the ambient air temperature, even if only a few degrees. Typical applications include the cooling system of compressors and condensers in chiller installations. ADVANTAGES AND DISADVANTAGES The advantages of dry cooling are:  No water and no water treatment equipment is required  Low maintenance requirements Relative disadvantages when compared to evaporative cooling are:  The lowest attainable temperature depends on the dry temperature of the ambient air. The dry air temperature is the temperature of the air measured with a thermometer freely exposed to the air but shielded from radiation and moisture.  A large heat exchanging surface between the ambient air and the intermediate cooling medium is needed.  The fans have a relatively high electrical energy consumption compared to those of a cooling tower ENERGY SAVING POSSIBILITIES ON DRY COOLING SYSTEMS  Because cooling systems are generally located outside, fallen leaves, bird nests, and other debris can obstruct free airflow through the heat exchanger. Regular cleaning of the heat exchanger and filters is necessary to maintain high efficiency.  The air which is drawn through the dry cooler should be as cool as possible so air intakes should be carefully placed to avoid any nearby heat sources such as warm gas exhausts.  The design requirement for a particular thermal power could be met by a small number of large fans, or by a larger number of smaller fans. The latter is more expensive to buy but more energy efficient, often resulting in a lower Total Cost of Ownership (TCO) over its life time.  The hot process fluid or gas should only be cooled as far as really necessary. The required electrical power is directly proportional to the difference between the air temperature and the temperature of the hot medium. If a final temperature of 40 °C is allowed, for example, it will be a waste of energy and money to cool the process fluid to 35 °C.  The output of the cooling installation can be controlled by a simple on/off control, by a variable frequency control of the fans, or by a cascade arrangement with on/off controls for each section. The choice and design of this control will have an important influence on the energy efficiency and TCO of the cooling system.
  • Publication No Cu0117 Issue Date: October 2011 Page 4 EVAPORATIVE COOLING This technique uses the latent heat of water vaporization to remove heat from the hot fluid or gas. At relative air humidity below 100%, water evaporates, absorbing an amount of heat known as the latent heat of vaporization and in this way cooling the remaining liquid or gas. The lower the relative humidity of the air, the more efficient the process will be. Relative humidity is measured using wet and dry bulb thermometers. The wet bulb thermometer is covered with a sock and kept wet—that is, at a 100% relative humidity—by means of a wick and a water reservoir. The dry bulb thermometer measures the temperature while freely exposed to the air, but shielded from radiation and moisture. The relative humidity of the air can be derived from the difference between the wet bulb and dry bulb temperatures using standard thermodynamic charts. On dry summer days when the dry bulb temperature is above 25 °C, the fluid can be cooled typically to temperatures around 21 °C. ADVANTAGES AND DISADVANTAGES Evaporative Cooling has the advantage of a better heat exchange compared to dry cooling, which results in:  A more compact installation (less ground surface needed)  Lower electrical consumption A disadvantage is the additional water cost. It consists of a water treatment cost and a cost for replacing water losses. The latter can be substantial with large cooling towers. COOLING TOWER TYPES There are three types of cooling towers:  Open cooling towers  Evaporative condenser and closed cooling towers  Hybrid cooling towers OPEN COOLING TOWER Figure 2: Example of an open cooling tower system. The water that needs to be cooled is sprayed in at the top of the cooling tower and falls due to gravity. Air, drawn upwards by the fan, makes contact with the falling water. The water partially evaporates absorbing heat from the remaining droplets. The cooled water is collected in a water reservoir under the cooling tower, ready to be returned to the process.
  • Publication No Cu0117 Issue Date: October 2011 Page 5 Figure 3: Schematic diagram of an open cooling tower. EVAPORATIVE CONDENSER AND CLOSED COOLING TOWER Figure 4: Principal drawing of an evaporating condenser. Evaporative condensers are integrated into many types of systems. The vapour to be condensed is circulated through a coil, which is continually wetted on the outside by a recirculation water system, similar to that of an
  • Publication No Cu0117 Issue Date: October 2011 Page 6 open cooling tower. Air blown into the tower causes a part of the water being circulated to evaporate, removing heat from the gaseous refrigerant in the coil and causing it to condense. The closed cooling tower has working principles similar to those of the evaporative condenser. The only difference is that the medium cooled in the coil is simply water, instead of a particular gaseous refrigerant. HYBRID COOLING TOWER A hybrid cooling tower can, depending on the external conditions, function in three different regimes:  Dry mode (like a dry cooler)  Adiabatic mode (like a closed evaporative cooling tower)  Dry-Wet mode (combination, which yields the maximum cooling performance) Due to the high initial price of the installation (roughly 5 times higher than an open cooling tower), hybrid cooling towers become interesting if the water price exceeds 1.5 EUR/m³. Hybrid cooling towers are mostly used when plume abatement is required. The emphasis for this technology is on saving of water rather than energy. ENERGY SAVING POSSIBILITIES IN THE EVAPORATIVE COOLING DOMAIN WHY IS A VARIABLE FREQUENCY DRIVE SO INTERESTING? The purpose of a fan in a cooling tower is to draw air through the tower so that the water can partially evaporate. This airflow should be controlled, depending on the heat load of the cooling tower and the ambient air temperature. Most fans on cooling towers are controlled either by using simple on/off control or by using a 2-speed motor. Depending on the average load of the cooling tower, substantial energy savings can be obtained using a variable frequency drive on the fan. For fans (as well as for pumps, etc.), the fluid flow is proportional to fan speed but energy consumption is proportional to the cube of fan speed. For those machines, the following formula is true: where P is the electrical power in kW and n is the number of revolutions of the fan This has important consequences for the energy efficiency. For example, by reducing the fan speed to 80% of the nominal flow, the power consumption will halve (i.e. 0.8 3 ). This can be accomplished by lowering the frequency from 50 Hz to 40 Hz. To accomplish the same flow (80% of nominal) using on/off controls would require an average power of 80% of nominal power. This means that in this situation, the variable frequency drive will consume 37.5% (3/8) less than a simple on/off control. The average saving potential of a variable frequency drive depends on the load pattern and the settings of the cooling tower. The more variation in the load, the more advantageous a variable frequency drive becomes.
  • Publication No Cu0117 Issue Date: October 2011 Page 7 OTHER ASPECTS INFLUENCING THE ENERGY EFFICIENCY The whole process of cooling depends heavily on the efficiency of heat exchange with the environment. Most water supplies are contaminated with other elements such as lime and organic material that can build up on the heat exchanging elements and reduce efficiency. Depending on the quality of the water source, a variety of water treatment measures will be necessary. Pumps need to be properly sized and controlled by variable frequency drives. The use of throttling devices should be avoided. As previously explained, cooling becomes more expensive as the required temperature reduces. Every degree of unnecessary cooling consumes more energy and water. For this reason, the required end-temperature should be regularly reassessed. Control systems that use bypasses to control the cooling demand are in no cases energy efficient.
  • Publication No Cu0117 Issue Date: October 2011 Page 8 COMPRESSION COOLING THEORETICAL AND ACTUAL CARNOT CYCLE Compression cooling machines are used in a broad range of applications, from household refrigerators to large industrial cooling systems. It makes use of a cooling refrigerant with a boiling point lower than the boiling point of water. The boiling point of a liquid decreased with reducing ambient pressure. By using compression and expansion, it is possible to vaporize a liquid refrigerant at a low temperature and condense it at a higher temperature. At the low temperature (evaporation temperature Tev), heat will be absorbed from the fluid which is to be cooled. At the high temperature (condensing temperature Tcd), heat will be emitted to the surroundings. Figure 5: Mollier diagram. Figure 5 shows a Mollier diagram representing the various states of the refrigerant during the cooling cycle. The main components of a compression cooling cycle are:  The compressor  The condenser  The expansion valve  The evaporator The most common type of compressor is the piston compressor, but other types have won acceptance, e.g. centrifugal and screw compressors. The piston compressor covers a very large capacity range, from small single cylinder models for household refrigerators up to 8 to 10 cylinder models with large swept volumes for industrial applications. The smallest applications make use of a hermetic compressor, in which compressor and motor are built together as a complete unit.
  • Publication No Cu0117 Issue Date: October 2011 Page 9 For medium to large plants, the semi-hermetic compressor is the most common. It has the advantage that shaft glands can be avoided, removing the need for a difficult maintenance operation. However, the design cannot be used for ammonia plants, as this refrigerant attacks motor windings. Still larger are Freon compressors and ammonia compressors, which are designed as ‘open’ compressors, meaning with the motor outside the crankcase. The power can be transmitted to the crankshaft directly or through a V-belt drive. THE CONDENSER The purpose of the condenser is to remove both the heat absorbed in the evaporator and the heat produced by compression. If the condenser cools the refrigerant further than necessary, this is called sub-cooling. One major advantage of sub-cooling is that the cooling capacity of the installation increases, as more heat can be absorbed in the evaporator. Moreover, sub-cooling prevents the formation of flash gas. This phenomenon takes place when the expansion valve is not fed with 100% liquid, but rather with a mixture of liquid and gas. This can be caused by:  Inappropriate condenser (damaged condenser fins or an inadequately-designed condenser)  A decrease in the condensing pressure in the system upstream of the expansion valve  Unwanted ingress of warmer ambient air into the conduit. Flash gas is a problem because it increases the volume of the mixture so that insufficient liquid can pass through the orifice of the expansion valve. Hence, not all the available surface of the evaporator is used and this causes instability of the cooling system. The presence of flash gas bubbles in the refrigerant can be observed through a glass eyelet placed ahead of the expansion valve. The disadvantages of too much sub-cooling are:  The capacity of the evaporator starts to decrease again from a certain level of sub-cooling  The evaporation pressure will decrease when the installation is lacking a proper regulator  The expansion valve operation becomes unstable. Many different kinds of condensers are available on the market. The shell and tube condenser is used in applications where sufficient cooling water is available. It consists of a horizontal cylinder with welded-on flat end caps that support the cooling tubes. End covers are bolted to the end plates. The refrigerant condensate flows through the cylinder, the cooling water through the tubes. The end covers are divided into sections by ribs. The sections act as reversing chambers for the water so that it circulates several times through the condenser. As a rule of thumb, the water heats up 5-10 °C with each passage through the condenser. A variant of this is the plate heat exchanger. If it is desirable or necessary to cut down on the amount of water, an evaporating condenser can be used. If no water at all is available for the condensing process, an air-cooled condenser must be used. Both types of condenser were explained in the previous chapter. THE EXPANSION VALVE As previously explained, the main purpose of an expansion valve is to lower the pressure of the liquid. Thermostatic expansion valves are the most common type utilized in direct-expansion refrigeration systems. It regulates the refrigerant flow rate to the evaporator according to the degree of superheating of the gaseous refrigerant leaving the evaporator. A thermostatic expansion valve consists of a valve body, a valve spring, diaphragm, and a sensing bulb. The sensing bulb is placed at the outlet of the evaporator and is connected to the upper part of the diaphragm by means of a capillary tube. If the temperature before the compressor is too high, it means there is not enough flow through the evaporator to satisfy the cooling demand. In this case, the orifice in the valve is enlarged to allow more refrigerant liquid to flow into the evaporator.
  • Publication No Cu0117 Issue Date: October 2011 Page 10 Electronic expansion valves can provide more sophisticated, effective, and energy-efficient flow control than thermostatic expansion valves. Currently, three types of electronic expansion valves are widely available: step motor valves, pulse-width-modulated valves, and analogue valves. Compared to the thermostatic expansion valves, the advantages of electronic expansion valves are the following:  They provide a more precise temperature control (better product conservation)  They provide consistent superheat control under fluctuating head pressure  They are able to be operated at low head pressure during lower ambient air temperature  The have a higher energy efficiency  They enable the use of a floating high-pressure control. Such a control will reduce the condensing temperature whenever possible, in this way increasing the efficiency of cooling installations. A floating high-pressure control gives better results with an electronic expansion valve than with a thermostatic one EVAPORATION SYSTEMS Many types of evaporators are available on the market, as various application-dependent requirements are imposed upon them. Evaporators for natural air circulation are used less and less because of the relatively poor heat transfer from the air to the cooling tubes. Earlier versions were fitted with plain tubes, but it is now common to use ribbed tubes or finned elements. Evaporator efficiency increases significantly with the use of forced air circulation evaporators. With an increase of air velocity, the heat transfer from air to tube is improved. As a result, a smaller evaporator surface can be used for a given cold yield. As the name implies, a liquid cooled heat exchanger cools liquid. The simplest method is to immerse a coil of tube in an open tank. Closed systems in which tube cooler designs similar to shell and tube condensers are employed, are increasingly common. MULTIPLE COMPRESSOR ARRANGEMENT Use of a single compressor to cool a cold storage room is not always the best solution. Indeed, a single compressor could be over-designed for the major part of its operational life. This causes the evaporation temperature to drop, with the following consequences:  Poor compressor efficiency  Short and frequent compressor runs  An increase of the drying effect at the evaporator side  More ice formation on the evaporator, requiring more defrosting cycles. In addition to all of the above, energy consumption will increase. For a well-designed installation, the following solutions can be considered: Multiple stage compression With a multiple stage compression system, bigger temperature differences (i.e. pressure ratios) can be achieved with reduced energy consumption. As an example, a cooling machine with a condensing temperature of 38 °C and an evaporating temperature of -40 °C, gives following results:
  • Publication No Cu0117 Issue Date: October 2011 Page 11  One stage compression: 100% energy consumption  Two stage compression: 80% energy consumption  Three stage compression: 77% energy consumption Because the initial investment cost increases with the number of stages, a careful analysis of all costs should be carried out. Parallel compressors, with one of them equipped with a variable frequency drive: One of the compressors can be equipped with a variable frequency drive. This compressor should be twice the size of the smallest compressor in the group, as it can only reduce its capacity to 50%. Advantages:  Very accurate control of the evaporating temperature  Limitation of the number of start-up cycles  High efficiency Disadvantages:  The compressor with capacity control will run most of the time  Higher initial investment cost (which pays itself back through lower energy consumption). EFFICIENCY—COP The efficiency of a chiller can be represented as the ratio between the thermal cooling capacity of the installation and the electrical power used by the compressor. The efficiency is expressed as the Coefficient Of Performance or COP. If an installation has a COP of 4, it means that for every unit of electrical energy, 4 units of cooling energy are produced. Because in reality there are several losses (heat and pressure), we have to multiply the COP of the theoretical Carnot compression cycle with a factor . This factor varies between 0.5 and 0.6 for a well-proportioned installation, but can go down to 0.2 in certain cases. From the previous formula, we can draw an important conclusion: the efficiency is higher when the condensing temperature is lower and the evaporation temperature is higher. The following table presents some indications for the COP for cooling systems used to cool liquids. The calculations are mostly based on the use of piston or screw compressors, but the values can also be applied to chillers with centrifugal compressors. For better comparison, the condensing temperature is held stable at 40 °C. Temperature In/Out describes the temperatures of the fluid to be cooled at the evaporator inlet and outlet.
  • Publication No Cu0117 Issue Date: October 2011 Page 12 Liquid Temperature In/Out (ºC) Thermal Cooling Capacity (kWh/m 3 ) COP Compressor COP System Electrical Consumption Compressor kWh/m 3 Electric system kWh/m 3 Water (pure) 13/7 6.98 4.79 3.88 1.46 1.8 Water (pure) 11/5 6.98 4.51 3.65 1.55 1.9 Mono ethylene 10% 4/-2 6.90 3.54 3.02 1.95 2.3 20% -2/-8 6.57 2.91 2.51 2.26 2.6 30% -10/-18 6.3 2,4 2.11 2.62 3.0 Mono propylene 10% 4/-2 6.92 3.54 3.02 1.95 2.3 20% -2/-8 6.85 3.06 2.51 2.24 2.7 30% -10/-18 6.8 2.57 2.11 2.64 3.2 Calcium chloride (CaCl2) 10% 4/-2 6.60 3.54 3.02 1.86 2.2 15% -2/-8 6.31 2.91 2.51 2.17 2.5 20% -8/-14 6.15 2.55 2.24 2.41 2.7 25% -14/-18 5.94 2.18 1.91 2.73 3.1 Table1: Indicative COPs for cooling systems. COPsystem takes into account all electrical power necessary to produce cooling (including fans and pumps), while COPcompressor only calculates using the electrical power consumption of the compressor. AMMONIA VERSUS OTHER REFRIGERANTS The design of refrigeration machines using ammonia is comparable with that of machines using halogenated fluids. The components, however, are made of ordinary steel instead of copper, because copper, copper alloys, and zinc are attacked by ammonia. Equipment adapted to ammonia is very specific and less widespread than its halogenated fluid type counterpart. Ammonia can be found in nature, but it is also synthesized in large quantities by the chemical industry. As a refrigerant, it has the following advantages:  Good thermodynamic properties (heat/mass transfer) resulting in machines with leading performance coefficients  A higher vaporization enthalpy, making it possible to produce temperatures as low as –60 °C  Chemical neutrality against components of the refrigeration system, excluding copper and its alloys, as well as reliability in the presence of humid air and water  Better stability against oil  Easy leak detection, even small leaks (olfactive detection at 5 ppm)  No emissions that affect the atmospheric ozone layer and no Greenhouse Gas Emissions  The lowest purchase price of all refrigerants, namely 5 to 8 times cheaper as halogenated fluid (but the installation cost will be higher because of the need for stainless steel)  Reduced pumping cost (embedded systems) and reduced piping dimensions for the same refrigerating power The restrictions associated with its use are due to the related hazards, in particular:  It is flammable, with an ignition temperature of 650 C  It is toxic at low concentrations in air (25 ppm)  The relatively high pressures require a higher pipe thickness than for halogenated refrigerants.
  • Publication No Cu0117 Issue Date: October 2011 Page 13 ENERGY SAVING POSSIBILITIES ON COMPRESSION COOLING Figure 6: Example of an evaporative chiller. The first and most important energy saving action is proper maintenance of the installation, including a regular cleaning of the condensers, a regular replacement of the compressor oil, and adequate defrosting of the evaporators. Other energy savings actions include:  Regularly checking the set point for the evaporation temperature. Efficiency increases with increasing evaporation temperature.  Regularly checking the set point for the condensation temperature. Efficiency increases with decreasing condensation temperature.  Opting for a centralized cooling system instead of several separate units. Bigger cooling installations run at higher efficiency than smaller ones (amongst other things because of higher performance of the individual parts).  Using evaporative cooling instead of compression cooling during wintertime. During the coldest months of the year, evaporative cooling can often achieve very low water temperatures (down to 5 °C).  Using cold storage to avoid or compensate for peaks in cooling load.  Equipping all pumps and compressors that have a reduced or variable load with a variable frequency drive.  In particular, installing a variable frequency drive on screw compressors. Screw compressors use a capacity slide that can reduce the capacity of the compressor down to 10%. This capacity reduction will be more efficient using variable frequency drives, as shown in the graph below.
  • Publication No Cu0117 Issue Date: October 2011 Page 14 Figure 7: The influence of varying cooling capacity on the power consumption, with and without variable frequency drive. 0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 Cooling capacity (% of nominal) Powerconsumption(%of nominal) capacity slide frequency drive Linear
  • Publication No Cu0117 Issue Date: October 2011 Page 15 CONCLUSIONS Cooling typically consumes 7% of electrical energy in Western Europe, and this figure is rising. Because it is such a large energy user, the design and application of the cooling plant should be carefully considered. Large energy savings can be achieved if certain general rules are applied:  Carefully assessing the cooling need to avoid over-dimensioning.  Choosing the right cooling technique. In some cases, it can be cost efficient to install two different systems; for example evaporative cooling for the coldest winter months and compression cooling for the remainder of the year.  Keeping the temperature differential low. For dry cooling systems, this means that the air intake should be located at a cold spot. In compression cooling systems, it is important to choose temperature set points as close to each other as possible while maintaining sufficient cooling capacity.  Carefully selecting and dimensioning equipment during the design phase. The cheapest is often not the most efficient.  Installing variable frequency drives on fans, pumps, and compressors.  Performing proper maintenance and cleaning actions on a regular basis. Further elements that influence the energy efficiency include:  Dry cooling o A large number of small fans are more energy efficient than a small number of large fans, but has a higher purchasing cost. An optimum can be calculated to achieve the lowest life cycle cost. o As dry cooling systems are generally located outside, a regular cleaning of the heat exchanger and the filters is necessary to maintain efficiency.  Evaporative cooling o The energy efficiency of the heat exchange will increase with decreasing contamination of the process water; best practice water treatment is therefore a crucial consideration. o Control systems that make use of a bypass to control cooling demand are in no cases energy efficient.  Compression cooling o One centralized cooling system will be more energy efficient than a number of smaller systems. o In some cases, the use of cold storage to compensate for peaks in the cooling load will be cost-efficient. REFERENCES [1] www.cti.org (Cooling Technology Institute), accessed October 2011 [2] American Society of Heating, Refrigerating and Air-conditioning Engineers Inc., Ashrae Handbook: Refrigeration (SI Edition), Atlanta (USA), 2002 [3] S.K. Wang, Handbook of air conditioning and refrigeration, McGraw-Hill (Second Edition), New York (USA), 2000 [4] European Commission, Integrated Pollution and Prevention Control (IPPC), Reference Document on the application of Best Available Techniques to Industrial Cooling Systems, 2001