Chilling - Notes

CHILLING & COOLING Needs: The techniques of water chilling and water recovery for plastics processing industry has reached new heights of sophistication that they are referred as means of increasing profits by means of water management. One needs evaluation of the equipment prior to purchasing the same – say what type of cooling equipment, where does it fit in properly, the water treatment systems and the water temperature levels that is to be maintained. Water Temperature Levels: Generally the water temperature level falls into two categories. 1. Equipment cooled with water obtained from a cooling tower or other evaporative cooling devices that operates between temperatures of 25º to 30ºC. The temperature cannot be brought down lesser than the wet bulb temperature in this type. 2. Most commonly used cooling type that needs refrigeration or chilling. The water temperatures in this type are 7º to 12ºC. A typical plastics process unit requires cooling water at more than single temperature. Thus, it may be necessary to have chilling unit for the molding process (for molds in injection & blow) and a cooling tower for hydraulic oil coolers, air compressors, condensers for chillers etc as it is economical. Heat Transfer Calculations: As to choose the cooling equipment, one need to know the amount of heat that is to be removed. This heat transfer calculation would guide us to know our requirement. Q = W x Cρ x dT. Q = the amount of heat is to be removed. W = the weight per unit time that is to be cooled, Cρ = specific heat of the material in question; and dT is the temperature difference of the material between entering and leaving conditions. Q is expressed in British Thermal Units per hour, Btu/hr. The other formula on the possibility of satisfying the requirement with wherewithal available at hand is like this: Q = U x A x LMTD. Q = Btu/hr; U = Overall heat-transfer co-efficient of the process; A = Area, LMTD is the Log Mean temperature difference between the mediums. The cooling of thermoplastics materials is the removal of all the heat that was put in. Since the process is continuous, the heat removal is expressed in Btu per hour or Btu/hr. This constitutes the heat load generally expressed as Q. The chilling unit must have this capacity. Considerations: Chiller capacities are mentioned as Tons of Refrigeration or TR. One ton of refrigeration equals a heat load of 12000 Btu/hr. However, while deciding on the size of the chiller capacity, one has to bear in mind the thermal losses in surfaces of the mold, cycling of the process, heat loss in atmosphere, interconnecting pipes and other parts of the system. As a general practice, 25% of additional rating is to be added to the capacity of chiller while deciding. Chillers can be either portable or stationary. Portable is preferred in smaller ranges. For larger capacity requirements, centralized chilling system is preferred. Chillers are of two types Air cooled and Water cooled. In comparison, water cooled systems have better efficiency on HP to Ton conversion basis and so Rupee per ton also gives good yield. The efficiency of air cooled chillers depends on the atmospheric temperatures. Water Load Determination: The flow through the process and the temperatures entering and leaving must be accurately observed as to calculate the cooling requirement.

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Flow: Flow can be determined from the performance curve graph of the pump or actual time testing method. From Curve graph: All the centrifugal pumps have similar characteristics. If the suction and the discharge pressures are known, it will be easier to plot the flow of the pump by plotting the same on the graph. However, one has to take several readings of the pressure installed on line at suction and discharge points. If the suction and discharge pressure is both positive, then the difference between the two is the pumping head. If the suction is negative and the discharge is positive, then the sum of the two is the pumping head. The pumping head can be converted into distance traveled. The load required to pump this water can be varied by changing the diameter of the impeller. One pump can have different impellers in same casing. As to ascertain the performance of the pump, the impeller diameter is very important. From measured type: One has to take a container of known volume and measure the flow of the water from the pump for a stipulated period. Or one can measure the empty bucket and weigh again with water. The difference is the weight of the water and the product of this weight with density will give the volume for the stipulated period. Here again, several tests should be conducted and the mean value should be considered. Nominal flow for design is 13.5 lpm per ton for cooling tower and 11lpm for chilled water. Cooling Tower: A cooling tower used the evaporative-cooling principle (adiabatic expansion) to cool water that has picked up heat from the system. After passing through the tower, the water is re-circulated into the system. Theoretically, water can be cooled equal to the wet-bulb temperature if sufficient surface area is available (this means infinite area is required). The wet-bulb temperature, in general, would be around 23ºC and the water can be cooled up to 25ºC. We cannot cool the water further down with evaporative cooling system. Due to evaporation losses, make-up water becomes necessary. The mineral content also is increased in the system water as the pure water gets evaporated and the mineral gets sediment. To neutralize one has to bleed the system that is to be made up again. Cooling Tower load Calculation: Chillers: In the event that the plant has got water cooled chillers, the condenser will be the part of cooling tower load. Condenser requires approximately 25% more of the chilling capacity (as due to additional heat put into refrigerant by compressor). Nominal cooling tower ratings are based on this figure. Hydraulic Drives: Generally, machinery manufacturers specify the flow rate for cooling the machine. Based on that information, one can calculate the load. The general thumb rule that applies for injection molding machine is we need 0.1 ton of cooling capacity per horsepower of load. Air compressors and dryers: Similar to hydraulic drives, air compressors also require 0.1 ton of cooling capacity per horse power. An additional 0.1 ton is to be added for after coolers if used. On screw compressors, the oil cooler load is 0.15 ton per horsepower and after cooler will be 0.05 ton per hp. Treatment of Water: Treatment of water is essential and should be considered a valuable adjunct to cooling systems. Non-treated water, resulting in scale deposits and corrosion, can be quite costly. In terms of production, a scale of 1mm will reduce cooling capacity by 30%. The thin deposits of scale, slime, algae in the system make the pump work harder by 15 to 50% increasing the power consumption and pre-mature wear of the impellers. 2 of 3

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In plastics processing line, the requirements are met with water softener or ion-exchanger. In this system, the magnesium and calcium ions are exchanged with easily soluble Sodium ions when the hard water is passed through the resin-bed (of softener) and brine solution (salt-water). This result in soft water that prevents carbonates scaling. Soft water is corrosive in nature and they are not in equilibrium state. Here one can add corrosion inhibitor to soft water. These inhibitors are polyphosphate chemicals those are commercially available. Slime and other organic growth like algae can be controlled by feeding biocides or bleaching of system. (Do not use powder as they are in carbonate form). Suspended solids can be prevented from entering into the system by means of on-line filters. In-process contamination of the system (like contamination from mold cooling channels, heat exchangers) can be prevented by having return-line filters. Silicate scaling can be controlled by having bleeding the system. This also dilutes the mineral content that was left out by the evaporation. Conclusion: While chillers earn us money, recovery system saves for us. In every case of cooling or chilling, we are concerned with heat balance. The input can be obtained from the amount of energy introduced to the system expressed in watts or horse power. The importance of getting all the capacity you pay for is particularly evident when the plastics process being cooled limits its heat-transfer efficiency.

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