A Seminar Report On water cooled chiller

A seminar report on “Opportunities for efficiency enhancement of water cooled chillers”
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  A seminar report on “O pportunities 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.  It’s 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 “Chiller”  as 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 25°C to -70°C [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-casting and 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 cooled Water 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 Power Consumption (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 is COP = Refrigeration Effect Work done (Higher COP means better efficiency)    The other commonly used and easily understood figure of merit is Specific 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 is EER = 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 occur i.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 45 0 C but it is further  cooled to 40 0 C, 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. Fig: Sub-cooling in vapour compression system [3] fig: T-S Diagram [3] From the T-S diagram we see that, The initial COP = T 1  T 1 -T 2 COP after sub-cooling, COP ’ = T 1 T 1 - T’ 2 As T 2 > T’ 2 , 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 T’ 2

San Krit Ization

Jul 29, 2017
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