Definition:  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

Since plastic part manufacturing whether injection, extrusion, blow rotational or any other process involves heating of the plastic pellets to a softening or melted state and then trying to cool said molten material into the correct shape, the characteristics of the cooling system are critical to economical processing.  In injection molding it has been stated that close to 90% of cycle time involves cooling. In extrusion, the maximum amount of through put is based upon being able to cool it. Though in the case of extrusion if one had enough floor space (in length) one can add more or longer cooling tanks to compensate for lack of cooling, though this is a capital cost and floor space cost,.

The typical plant cooling is made up of a supply of fluid, reservoir for fluid, a way to temperature control the fluid, heat exchangers, pumps, piping, fittings, valves and hoses to the tooling, trough, sprayers or other such components  that control /supply / regulate the cooling of the plastic. Finally after use in the application / process of cooling the fluid is then transported back to the temperature control location for the fluid, to be reconditioned and returned to the process.  The only time it is not is if one were to use a supply that is endless and dumps into drain or lake / pond.  What is being discussed is a closed loop cooling system for a plant.  

Some basic rules of thumb to start:

1)      fluid (water) takes the path of least resistance

2)      flow is a critical function of our cooling medium

3)      Pressure may be used to monitor the operation

4)      Area of the flow channels needs to be known

5)      Turbulent flow is desired at the heat exchanger interface, whether in temperature control of the fluid and or than of the mold, calibrators or other interface to the plastics.

6)      Turbulent flow is a number calculation based on Reynolds number of greater than 4000

7)      Laminar flow is slow moving and based on Reynolds number below 2300

8)      Reynolds number is based on a mathematical equation.

9)      A chiller ton is based on 12000 Btu's / hour (meaning it is capable of removing 12000Btus' per hour) A 2 ton chiller would have a capacity of 24000 Btu's / hour.

10)   A chillers capacity is based on a 10 degree delta T and a set point 50F or above, meaning that if the unit supplies 55°F fluids to a system the return temperature is good if it goes up to 65F.

11)   Delta "Δ" means a difference in two measurements. In the case of chiller or tower, a supply and return temperature is expressed in Delta T, for pressure it would be stated Delta P.

12)   A chiller is typically rated at 50°F. For each degree below 50°F a loss of 2% of rated capacity is experienced on the chiller. Meaning that a set point temperature of 45°F on a 10 ton chiller equates to 2% multiplied by  5 equaling  a 10% reduction meaning that the 10 ton of cooling capacity is now reduced to 9 tons.

13)   A tower ton is based on 15000 Btu's

14)   BTU (British thermal unit) is the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32 to 212°F (0 to 100°C)

The above listing of ROT (rules of thumb) is not complete and further rules shall be brought forward as this series continues.

To be continued;

PC 1 10072012.1

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