Therefore check, who will be eternally bound

Air Conditioning Refrigeration Chillers Therefore check, who will be eternally bound Lars Keller, Munich, Ralf Beleth, Karlsruhe This comparison of various chiller units and associated heat exchangers should provide help in determining on a project specific basis the optimal machine. The general rule is that all relevant factors be taken into account for the application, and to carry out a weighting and evaluation. Rating the Efficiency To assess the efficiency of chillers, the valuation by Eurovent and ARI are widespread. The ratio of nominal cooling capacity to electrical power consumption of the compressors and the power consumption of all control and security measures, is according to Eurovent defined as EER (ENERGY EFFICIENCY RATIO) and is the basis of classification from the most efficient "Class A" extending to the most inefficient "Class F". The ESEER (EUROPEAN SEASONAL ENERGY EFFICIENCY RATIO) according to Eurovent and IPLV (INTEGRATED PART LOAD VALUE) according to ARI Standard 550/590 2003 makes a statement about the efficiency of chillers at full and partial load with decreasing coolant temperature. Autoren Dipl. Ing. (FH) Lars Keller as Head of Sales / Marketing at Kältetechnik aircool GmbH and author of numerous technical publications and the book Guide for Ventilation and Air Conditioning. This is true, not only in life, but also in the purchase of a new chiller. Watercooled chillers with capacities ranging from approximately 2 MW are currently available in various configurations. Compressors are of the semihermetic screw or turbo type, the refrigerant R134a and R410A are prevalent, and shell and tube heat exchangers will be installed. In the cooling system, the operator is faced with the decision to use dry coolers, Hybrid coolers or cooling towers. But to get a reliable statement concerning the anticipated cost of operation, the expected load characteristics have to be determined and calculated. This must be done for the entire system (chiller, pumps and heat exchangers), as a onesided optimization of the chiller does not necessarily minimize the total cost of operation. Here, the following criteria are to be observed: The lower the cooling water outlet temperature at the condenser of the refrigeration unit, the more efficiently it is operating and it provides more capacity. (Vendor Specific limits are to be observed, as a minimum pressure ratio pc/ po is necessary) The smaller the difference between the cooling water outlet temperature of the cooling tower and the wet bulb temperature, the higher are investment costs and space requirements of the cooling tower. The greater the cooling water temperature spread, the lower the flow rate, pump capacity and pipe dimensions, practice has shown that 5 to 8 K to be an economic size for the entire plant. Dipl.Ing. (FH) Ralf Beleth is a sales representative at the KTK Kühlturm Karlsruhe GmbH Which is the correct refrigerant? For years, the monochlorinefree refrigerant R134a has established itself due to its favorable thermodynamic properties. The advantage is seen in the low differential pressure pcpo and the high critical temperature, which is not achieved during operation. Turbo chillers are always operated with R134a. The nonazeotropic refrigerant R410A is a mixture of 50% R125 and R32, with a negligible boiling range at a phase change of less than 0.2 K. The disadvantage is the high pressure needed, which places enormous demands on the screw compressor technology. Due to the high volumetric cooling capacity, less Refrigerant charge is needed and smaller compressors are necessary than for R134a. To illustrate, here an example: A screw compressor with R134a as refrigerant provides 100 kw cooling capacity, by using R410A it will provide about 230 kw! This results in compact equipment dimensions and low cost. The ODP (Ozone Depletion Potential) makes a statement about the destructive effect of ozone, the refrigerant and data are based on the CFC R11 with ODP = 1 The GWP (Greenhouse Warming Potential / Global Warming Potential) represents the global warming potential of a substance as opposed to CO2. The GWPfactor makes a statement about how many times stronger in comparison to CO2, the direct contribution to the greenhouse effect is over a specified time horizon of 100 years. For example, if 1 kg of refrigerant R134a is emitted into the atmosphere, this is equivalent to an emission of 1300 kg C02. 40 HLH Bd. 60 (2009) Nr. 10 October

Air Conditioning Refrigeration R134a R410A Volumetric cooling capacity 0 /40 kj/m 3 2.050 4.781 Evaporation heat kj/kg 216 268 Critical temperature C 101 71 Absolute pressure at 0 C(p o ) bar 2,93 8,06 Absolute pressure at 40 C (p c ) bar 10,17 24,36 Differential pressure p c p o bar 7,24 16,3 Pressure ratio p c / p o 3,47 3,02 Temperature glide K 0 < 0,2 ODP 0 0 GWP (relative to CO2, 100a) 1.300 1.720 OEL (occupational exposure limit) kg/m 3 0,25 0,44 The TEWI (Total Equivalent Warming Impact) takes into account the sum of direct (GWP contribution of refrigerant leaks during installation and disposal) and indirect (contribution of CO2 emissions resulting from energy consumption to operate the plant) emissions of greenhouse gases. If the refrigerant loss is minimized, then the influence of the GWP in the TEWI is very low. A comparison of the main physical data is shown in Table 1. Turbo or Screw The semihermetical screw compressors are in the power range up to 2200 kw, depending on the refrigerant, the operating conditions and number of compressors. They show a wide range of operation, they can be used with the default configuration; varying flow temperatures of 8 C to 15 C with cooling water outlet temperatures of up to 55 C. The performance adjustment should be continuously adjustable from 25 to 100% of the standard cooling capacity, a flow temperature control is to be considered as prior art. Compressors are started via the stardelta, to reduce the starting current a soft start can be used, more recently, frequency converters are used. McQuay semihermeticturbo compressors are used in the power range from 900 to 4500 kw. Since a maximum of two compressors can be used in a refrigeration circuit, a capacity of up to 9 MW can be achieved. Each turbo chiller is based on a projectspecific configuration; the selection of the main components such as motors, compressors, gears and gear ratios, impeller and heat exchangers is dependent on the operating conditions. For this reason, as well as flooded evaporation, which requires an exact Refrigerant charge, varying operating points Table 1 are not quite as arbitrary as with screw chillers and must be verified for the configuration. The adjustable power control from 10 to 100% is best carried out using the inlet guide vane setting via oil pressure. The turbine can be started via star / delta or softstart, by mounting an optional VFD (Variable Frequency Drive, frequency converter) corresponding to the starting current, respective to the operating current (Figure 1). Again, the VFD will significantly contribute to an increase in the ESEER / IPLV, through the loss dissipation of the frequency converter; however the 100% EER decreases by about 2.5%. In Table 2 a comparison of different Chillers at different operating points and cooling systems can be seen. Cooling Towers When it comes to cooling towers everyone has probably one of those big steaming power generation plant cooling towers in mind. For a large portion of the cooling tower processes, standard cooling towers are implemented. However, they perform their job in Physical Data for the refrigerants R134a und R410A most instances well concealed and can only be seen when taking a closer look at the roofs. Often, cooling tower issues of are placed in the background, even with respect to: The relatively small cost compared to the cooling system, although the choice of an appropriate process, system, tuning of the refrigeration unit and integration into the overall system has a significant impact on the efficiency of the overall system. Distinction between the methods of heat transfer to the outside Basically, two methods are used to dissipate the excess heat to the environment. Dry cooling transfers the heat to the surrounding air by the temperature gradient of the coolant to ambient air temperature. Accordingly, the high temperature of the cooling medium of approx. 45/40 C is assumed when designing for the Picture 1 2.5 MW turbo chiller WSC 100 with freestanding frequency converter of protection IP54 to improve partload efficiency at decreasing coolant temperature. HLH Bd. 60 (2009) Nr. 10 October 41

Air Conditioning Refrigeration Chiller Specifications Unit Proximus Evolution 596.2 XE S T WSC 100 WDC 079 Cooling capacity kw 2.268 2.196 1.933 2.200 2.200 2.200 2.200 2.200 2.200 Power consumption at the terminal box kw 434 461 553 310 352 509 314 352 523 Condenser capacity kw 2.702 2.657 2.486 2.510 2.552 2.709 2.514 2.552 2.723 Cold water temperatures C 12 / 7 12/ 7 12 / 7 Cooling water temperatures C 27 / 32 30 / 35 40 / 45 27 / 32 30 / 35 40 / 45 27 / 32 30 / 35 40 / 45 Medium Water 34% Glycol 34% Glycol Water 34% Glycol 34% Glycol Water 34% Glycol 34% Glycol Performance figures 5,2 4,8 3,5 7,1 6,3 4,3 7,0 6,3 4, 2 ESEERValue 5,38 6,21 7,48 Evaporation dry flooded flooded Number / type of compressors (Units) Quantity 2 / semihermetic screw compressor 1 / semihermetic turbo compressor 2 / semihermetic turbo compressor Capacity control at constant cooling water temperature % 12.5 100 10 100 10 100 20 100 5 100 5 100 20 100 Length x Width x Hight mm 4.800 x 1.350 x 2.550 4.300 x 2.100 x 2.550 5.600 x 1.800 x 2.550 Refrigerant circuits Quantity 2 1 1 Refrigerant R 410A 134a 134a Refrigerant charge amount kg 2 x 130 636 670 670 884 920 878 Cooling Circuit Technical Data heat exchangers Design temperature Unit C Evaporation cooler for an open circuit with 2 x KD 2/1828S2 Evaporation cooler for a closed circuit with 2 x KI 3/122812 Dual cooling system / hybrid coolers for closed circuit with axial fans 3 x KA VH092x6 Chiller for a closed circuit with 3 x KA VL092x6 Evaporative c ooling Evaporative cooling / Dry c ooling Dry cooling open c ircuit closed circuit closed circuit wet bulb 21 C wet bulb 21 C wet bulb and ambient air temperature 21 C / 18 C ambient air temperature 32 C Cooling agent Water Waterglycol mixture 34% Waterglycol mixture 34% Cooling water temperature C 32 / 27 35 / 30 45 / 40 Cooling c apacity kw 2.700 2.500 2.650 2.550 2.650 2.550 2.700 2.500 Ventilator shaft capacity max. kw 20,8 48,44 86,4 90,00 Type of water q uality Basis VDI 3803 Basis VDI 3803 Osmotic water Additional water conditioning necessary hardness stabilization, corrosion yes yes no protection, organic load Evaporation water quantity max. (full load) m³/h 4,0 3,9 3,9 Water discharge reference value m³/h 1, 3 1,3 0,6 Sound output level with out / with additional sound elimination measures db(a) 99 / 78 105 / 84 108 / 99 / Dimensions Length x Width x Height mm 2 x 5.000 x 3.700 x 2.400 2 x 5.000 x 3.700 x 2.900 3 x 9.300 x 2.250 x 2.400 3 x 9.300 x 2.250 x 2.370 Floor space approx. (without maintenance area) m² 37 37 63 63 Operational weight cooling towers kg 12.800 29.000 24.600 12.900 Table 2 Technical Data of various refrigeration units and heat exchangers typical maximum surrounding air temperatures in the summer of about 32 to 34 C in Germany. In wet or evaporative cooling, one takes advantage of the physical effect that for a corresponding change in the aggregate state, vaporization energy is required. Thus evaporative cooling towers have a considerably greater efficiency of heat dissipation because of latent heat. Compared to dry cooling, significantly lower the temperatures of the cooling medium can be achieved, since the ambient air temperature is not critical, but the wet bulb temperature is. Thus cooling media temperatures of e.g. 32/27 C can be achieved in an implementation for a wet bulb temperature of 21 to 22 (Fig. 2). Dry or glycol coolers are often given preference over evaporative cooling, because it avoids the hassle and cost of water consumption and water treatment. For decentralized and relatively small plants dry cooling may be the choice, but in large and centralized systems, evaporative cooling is to be preferred. 42 HLH Bd. 60 (2009) Nr. 10 October

Air Conditioning Refrigeration A comparison of space and power consumption can be seen in Table 2. However, in any case the choice between dry / evaporative cooling, the impact on the chiller should also be taken into account. The example of the turbo chiller WSC 100, the clamp power consumption when using a dry cooler is 509 kw, when using an evaporative cooling tower, however, the power consumption is only 310 kw: Thus just by the choice of implementing another cooling process the electrical energy used by the chiller can be reduced by 40%! Distinction based on the cooling medium system Here one can also distinguish two basic systems. Certainly the oldest and best known method is that heated water is exposed directly to the ambient air, and by way of evaporation of part of the water, cooling of the same is achieved. Cooling towers where the water is directly exposed to the ambient air are designated as open cooling towers (open cooling water circuit). The earliest cooling towers emerged at the beginning of industrialization in the form of natural draft cooling towers which were relatively simple wooden structures. Graduation houses are in use today for another purpose but operate on the same principle. The most efficient heat transfer is achieved by way of direct contact of water with the ambient air. However, the water is exposed to contamination in the air and adds to the impurities such as water dissolved minerals are thereby concentrated, since only pure water in its vapor phase may leave the circuit, this should be taken into account in plant design, e.g. through a impurity factor in the chiller. If this qualitative change in the water can not be accepted, protection may be achieved by separation of the circuits to be cooled machine / system from the influence of the polluted water. If it is realized directly in the cooling tower loop separation, usually by the installation of a tube bundle heat exchanger), it is known as a "closed cooling tower" (closed cooling water circuit). Which is the cooling tower that you would like to have? To begin with, this question can not be answered here. If the enduser / operator wants to have an efficiently operating system, the cooling tower manufacturers will be happy to provide a detailed consultation. In recent decades, a wide variety of different types of cooling towers for various applications have been developed, which are described below. Open cooling towers with They are well suited by their compact design for use in a small space, are of low weight and move large amounts of heat with as little energy as possible. The offer the ability to mount additional silencers, so that even demanding noise requirements can be met. Because of the mode of operation it is obviously not possible to use dry cooling, which means that over the entire operating life one must expect correspondingly high water consumption. However, open cooling towers may be used at sufficiently low temperatures for "free cooling" (i.e. cooling without refrigeration machine operation). The minimum cooling water temperature usable with free cooling, however, is limited due to the risk of ice formation at below10 degrees C. Closed cooling towers with The tube bundle heat exchangers of the cooling water circuit is protected from pollution, but the required space, weight and energy requirements are higher than for an open cooling tower. Silencers can be mounted, as well as the operation for free cooling. Additionally, it is possible to run the cooling tower in dry mode. In observance of the cooling water temperatures and the ambient air temperature, the tube bundle heat exchanger in dry operation, can dissipate approximately 10 to 20% of the designated capacity of the chiller in operation. An application example is the air conditioning of a building, which in the transitional period and in the winter only one server room needs to be cooled. HLH Bd. 60 (2009) Nr. 10 October 43

Air Conditioning Refrigeration Dual heat exchangers or hybrid coolers The aim of these units is to combine the benefits of evaporative and dry cooling in one device. Thus it is possible on the one hand, to reach the low temperatures of an evaporative cooling, and on the other hand through the rational selection of the switching point between wet and dry operation of approximately 15 to 18 C to achieve considerable savings of water. Lower set points worsen the water savings and thus the cost savings. The devices are designed so that even at the switchover point, the full cooling tower capacity is available. These systems with intelligent and effectively controlled equipment also have their price. Dry coolers or "glycol" These devices have a relatively simple structure, consisting of a tube bundle with slats designed to achieve the greatest possible heat transfer surface area. Fans provide the necessary exchange of air. Of course you do not need any extra water, but usually have higher electric power consumption than an evaporative cooler and, as already mentioned, the higher coolant temperatures have a negative impact on the performance numbers of the chiller. Advantageous is the ease of installation, the closed cooling circuit and the high potential for free cooling capacity. Water preparation and treatment for evaporative cooling towers Evaporative cooling towers can fulfill their purpose, only when the water quality is good. The best cooling tower will fail, if insufficient attention is paid to the manufacturer's guidelines for water quality. The currently available equipment e.g. for water softening, desalination are relatively easy to handle. When designing the system, specialized firms will provide appropriate guidance. Energyefficient operation of cooling towers Because a cooling tower is usually designed for the highest summer temperatures (e.g. wet bulb temperature of 21 C) it is at all lower temperatures actually oversized. The resulting Picture 2 hx diagram, a representation of the process of dry and wet cooling. In the initial state at the entry point of the air is the same. Through the various processes involved the corresponding line results. "extra" power is not really needed. The performance adjustment can be effected by regulation of the airflow through the cooling tower. For a long time, the fans were operated with motors having 2 or sometimes even 3 switchable speeds. Thanks to the fact that the frequency converter which is only slightly more expensive than the contactor control circuit for a speed switch, they are frequently implemented and for energyefficient operation they are the best choice. In connection with the control of the chiller by use of a frequency converter, a continuously variable cooling water temperature can easily be achieved. Conclusion A blanket statement, which system version should be implemented is not possible, the project relevant factors such as capital and operating costs, efficiency, redundancy, reliability, setup time and situation, plant layout, ambient temperatures, and noise standards, as well as (to name but a few) must be taken into account. To find the ideal chiller / cooling tower combination on the basis of investment and operating costs, in preparation a load profile must be created, which also provides information on the required partial load performance of each unit. Frequency converters provide low starting currents and high ESEER / IPLV values, and at constant cooling water temperatures the operating cost savings are not so great. Basically, it is necessary to determine whether heat recovery and / or free cooling can be implemented, the performance numbers (benefits / costs) which can thus be realized, cannot be achieved by any of the most efficient compression refrigeration machines! 44 HLH Bd. 60 (2009) Nr. 10 October