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MaintenanceCircleTeam

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April 30, 2007

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NEWSLETTER FOR MAINTENANCE COMMUNITY Word for the day: TR

Today, it is nearly impossible to imagine an office, a shopping-mall, a high-speed machine, an automobile without an air-conditioner or a chiller. It’s one of the greatest inventions of mankind which finds its application virtually in every walk of life. Preserving food, meat, medicines and other perishable goods has a history of 2000 years. China, India, Rome people used ice for various purposes and the process was known as “ice harvesting,” which even today is in practice in many cold countries. Refrigeration is a simple process of removing “heat” from one area and rejecting it to another. So, if one area gets colder due to “refrigeration” effect, another area – usually nearest open atmosphere – gets hotter and the cycle continues. Independent of the type of refrigeration – heat removal – process used, TR – short form for Ton of Refrigeration –is the most commonly used measure of specifying its “cooling” ability. The origin of TR goes back to “ice harvesting” period and hence still is used in practice. One Ton of Refrigeration is defined as the amount of heat that is absorbed by one ton – 1000 kilograms – of ice at 0°C before it completely melts to water at the same temperature in 24 hours. Defined other way, One Ton of Refrigeration is the amount of heat that has to be removed from one ton – 1000 kilograms – of water at 0°C to completely freeze it into ice at the same temperature. Scientists wanted to measure this “heat” more precisely and over a period of time, came with two widely accepted units: BTU – British Thermal Unit – and Kcal – Kilo Calories. Let us understand each of them and relate them to TR in practical sense. The amount of heat required to rise temperature of one pound – 1 lb – of water by 1°F is 1 British Thermal Unit (1 BTU). Similarly one calorie is defined as the amount of heat required to rise temperature of one gram of water by 1°C. A larger unit is kilocalorie – Kcal – which is the amount of heat required to rise temperature of one liter of water by 1°C. But we all know that water starts boiling at 100°C and starts becoming steam. Therefore, the amount of heat which converts one pound of water completely into steam is called Latent Heat of Vaporization and it is equal to 970 BTU per pound (540 Kcal per Kg). Similarly, the amount of heat necessary to convert one pound of water at 0°C completely into ice is called Latent Heat of Fusion and it is equal to 144 BTU per pound (80 Kcal per Kg). Now let us give some values to One Ton of Refrigeration. Latent heat of Ice (Latent heat of fusion): One Ton: Heat to be removed, in 24 hours:

144 BTU / Pound 2000 pounds 2000 x 144 / 24 hours = 12000 BTU / Hour

=

80 Kcal / Kilogram 1000 Kilograms 1000 x 80 / 24 hours = 3333 Kcal / Hour Rounded to 3000 Kcal / Hour

Therefore, One TR = 12000 BTU / Hour = 3000 Kcal / Hour So every air-conditioner, cooling tower, HVAC (Heating Ventilation Air Conditioning) system is usually specified in TR capacity. The TR required also depends upon three major factors: (a) Present inside temperature (b) Required inside temperature (c) Outside temperature. For example, 1, 1.5 and 2 ton ACs are commonly used in houses, small offices. As a thumb rule, 1 ton of refrigeration can cool 400 square feet area (assuming 8 to 10 feet roof height). From international conversion table, it is found that 1 kilowatt of electrical energy is equivalent to 860 kcal / hour (0.29 TR) of thermal energy. Let us consider 3 instances to approximately calculate the TR required. Accurate calculations will be done by Refrigeration experts referring to various standard charts.

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.

MaintenanceCircleTeam

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April 30, 2007

An office room measuring 12 feet wide, 20 feet long and 10 feet high needs to be comfortably cooled. How much TR will satisfy the requirement? Room Volume = 12 x 20 x 10 = 2400 cubic feet = 2400 / 400 (from thumb rule) = 6 TR A machine generating 12 Kilowatts of electrical heat needs to be cooled for its proper functioning. How much TR is required to complete the task? From conversion table, 1 kilowatt = 860 kcal / hour; 12 kilowatts = 12 x 860 = 10320 kcal / hour = 10320 / 3000 = 3.44TR ( or 4 TR) is necessary to remove the necessary electrical heat. A hydraulic machine has a 40 kilowatt motor fitted to its power block. What should be the cooling tower capacity to cool the entire oil on a continuous cycle? 40 kilowatt = 40 x 860 / 3000 = 11.5 TR (or 12 TR) cooling tower should be selected to completely remove the heat generated from this hydraulic machine. After electric motors, air-conditioning systems are the largest consumers of power in the world. With advancement of technology and energy efficient designs, there is considerable reduction in power consumption. A general air conditioning system today consumes approximately 1.25 to 1.4 Kilowatts per ton of refrigeration. For instance, if 1 TR air conditioner is installed in an office, it will consume 1.25 to 1.4 units for every hour of its operation. Due to increased usage of air conditioners (refrigerants), ozone layer of the earth is getting thinner day by day leading to a serious phenomenon called “Global Warming.” Because of the reduction in ozone layer thickness, harmful ultraviolet rays from Sun will directly hit the earth and increase its temperature. The results will be hotter summer, less rainfall and severe water shortage. Following are some of the guidelines which can be followed to reduce various negative effects of air-conditioning. z z z z z z z z z z

Use non-CFC type refrigerants – R134a for example – as much as possible Insulate all the pipes carrying cold air or cold water Do not block any ducts of air conditioning system Clean the filters thoroughly and avoid dust accumulation Evaporator radiator should not be dirty (especially if it is out door type) Regularly check refrigerant pressures in the system It is suggested to select a higher capacity and switch if off rather than selecting a lower capacity and overloading it Monitor compressor conditions on a regular basis If installed for a machine, switch off the units when machines are not running If installed for a room, try adding double-door system or air-curtains to avoid external heat from entering the air conditioned area

From maintenance point of view, couple of data can be collected to monitor the air-conditioners’ health. Firstly, measure the current drawn by compressor when AC is new. Then on a regular basis – monthly or bi-monthly – measure the same and compare the values (you may also create a nice chart showing the trend). Secondly, preferably install an hour meter to the compressor. Collect number of hours compressor remains ON data and monitor them on a regular basis. For instance – assuming almost same external weather conditions – if compressor runs longer to achieve the same temperature than before, there should be some problem, which must be addressed immediately. In Indian sub-continent, Bureau of Energy Efficiency – BEE, Government of India for short – certifies various airconditioners, centralized systems with their “Star Rating” which can be looked into, before buying the product. This will help in saving precious energy and ozone, since the products will be tested for their energy efficiency and refrigeration ability. Put it simply, we will consume less KWh per TR!

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.