Definitions: Plant: meaning a facilities / operation where manufacturing is occurring.  Cooling: 1) free of heat 2) capable of lowering temperature.  System: an assemblage or combination of things or parts forming a complex or unitary whole: a cooling system. Calculations: 1) an estimate based on the various facts.  2) The results or product of calculating

In reviewing plant cooling system one has to understand the basics, meaning that within a molding, extrusion or any processing plant, cooling is a key to the process. The primary function of cooling is the maintenance of temperature, within the process.  It should be stated that the main unit of exchange is via heat exchangers, which in fact are the molds, oil cooling, thermulator, cooling tanks (for extrusion) and other methods, some of which just spray the cooling fluid onto the steel mold, or plastic parts. In addition we have to maintain a heat exchanger for hydraulics, feet throats, and other auxiliary equipment.

In a heat exchanger heat is conducted from one substance to another and then another. In the case of molding the heat is in the plastic material and then via contact is conducted through a metal which than adsorbs the heat but at the same time this heat is taken away by a cooling medium (water channels) thus the mold temperature is maintained at hopefully a constant temperature.  Thus we know the heat being supplied by the plastic and we know the amount of plastic being processed per hour.  What now needs to be done is to calculate the energy in a unit such as Btu's. To do so requires the use of the energy content of the plastic.

An example might be processing HDPE at a processing temperature of 425°F (218°C) which has approximately 369 Btu's per pound. If one were to process 40 pounds per hour at 425°F than one would have 14,760 Btu's of energy and that divided by 12,000 Btu's/ton equals 1.23 tons of cooling.  The point here is that this is all specific to 425°F melt temperature, and if temperature is changed than the Btu's needed would be changed as the Btu's are based on a constant per degree F. 

Note that many of the equipment suppliers have created charts which list how many pounds an hour can be processed per ton of cooling. Thus there is a factor of safety included in their calculation but it now makes it easier. For example in the calculation above at 40 pounds per hour by looking up HDPE on the chart it states 30lbs/HR = 1 Ton. Thus dividing 40 /30 = 1.33 Tons, understand this takes into account other factors, such as friction heat by the pump and piping. 

The first step to calculating for the size of the system is to start listing the worst case (largest through put) of material per each machine in the factory.  This is the minimum number, as one could also use a factor supplied by the machine manufacture of though put, but this would create an oversized system, this due to the way though put is calculated.

The second step is to look at other areas that cooling is to be used.  This is the feed throats of machine, which in effect require 0.5 tons per throat.  Other areas of the machine also include the cooling of the hydraulics, which is than based on the pump motor size and for each Horsepower is 0.1 Ton.

The third step is to look at any auxiliary equipment one is cooling, such as chillers which may be water cooled, temperature control units, and or screws and barrels in some extrusion applications along with gear boxes.

Once one has listed everything that is to be cooled, (temperature controlled) the calculations can be totaled. The issue now becomes is one running a tower and a chiller system or only one or the other. Remember that a chiller is rated at 12,000 Btu's per ton and a tower is rated at 15,000 Btu's per ton.

This distinction between a tower and a chiller can be explained as a tower is good for supplying water temperature above 80 F (26C) to the plant, and the temperature can fluctuate. This is due to the method of cooling through evaporation, and environmental temperature which can affect this result.

A chiller is capable of supplying water from 70°F to down as low as 20°F if the fluid is proper, but typically is supplying 50 to 70 degree water to the process. Also note that if process temperature is below 50°F than for each degree below there is a 2% drop-off in chiller capacity. An example is that if a chiller were rated at 5 Tons but was to process fluid at 40°F this is a 10 degree decrease from 50°F thus multiplied by 2% we get a 20% decrease in tonnage or now the 5 ton chiller is really only capable of 4 Tons of cooling.  

The calculation for plant cooling tonnage is mathematical  in that one adds all the materials to be processed per hour by type, calculates out the tonnage and then all the other needs for equipment, and this is the tonnage requirement for the plant.

The following is a start of data for calculation:

(This is a summary of data collected, when two rates are listed this means that data was different per manufacture of cooling system)

PC 2 23072012.01

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