To heat or cool resin in the mold, processors traditionally use water as the heat-transfer fluid for temperatures below 212°F and oil for higher temperatures. However, water under pressure has a boiling point higher than 212°F, making it also suitable for higher-temperature applications. The decision to use oil or water is commonly based solely on temperature with little or no consideration given to the actual characteristics of the heat-transfer fluid.
It is important to determine whether water or oil provides better heat transfer. The heat-transfer capacity of water versus oil can be evaluated by quantifying heat capacity. Heat capacity is equivalent to the product of the mass to be heated times the specific heat of the fluid times the differential temperature, in F° for the duration of the heat-up.
These are typical one- and two-zone microprocessor-controlled temperature control units for water temperature control up to 320°F.
Each gallon pumped will transport the fluid’s heat capacity per gallon. Water has an average specific heat (Cp) of 1.0 Btu/lb mass (expressed in °F), while heat-transfer oil has an average specific heat (Cp) of .5 Btu/lb mass (expressed in °F).
Water will carry twice as much heat as the same quantity of oil. Heat-transfer effectiveness requires that the fluid flows easily through the system. Water has a density of about 8.3 lb mass/gal, whereas oil has a density of 7.25 lb mass/gal. This means that you pump 1.05 more lb mass per gallon of water than for a gallon of oil, which indicates superior heat transfer.
One of the most significant differences between water and oil is thermal conductivity.
Water can conduct more than 4.7 times the heat per square foot of heat transfer surface as oil. In other words, water offers significantly better heat transfer than oil for the same volume. Note that the mold can not be purged until the temperature is reduced to below boiling. This can occur more quickly with water than oil.
Although water can be used at higher temperatures, there is a practical limit based on equipment design. We have identified 320°F as an appropriate cutoff for water vs. oil.
A hot water system operates at an elevated pressure to prevent boiling. This pressure is relatively low compared to the operating pressures of an injection molding machine but could present a hazard without certain precautions.
For example, if there were to be a leak of the very hot, pressurized water, it will turn to steam upon contact with air. The heat of vaporization, the Btu/lb mass necessary to change the water from a liquid to steam, is about 970 Btu/lb mass. This energy is lost from the fluid as the water turns to steam and reduces the heat rapidly.
On the other hand, oil will not normally “boil” or vaporize if it leaks. The hot oil carries all of its heat with it to contact the surfaces impinged on by the leak. Even small amounts of oil may cause burns.
Different pumping powers are necessary to move less-viscous water versus more-viscous oil. In terms of a water system, the design must accommodate the higher pressures. This is true for both the tank and the hoses, which ironically are the same high-pressure and -temperature hoses used in hot-oil systems. Also, the ability of the system to cool quickly reduces the time required to reach a temperature where the mold can be safely evacuated.
Water systems designed for use over 212°F require leak-proof pumps. This can be achieved with magnetically coupled pumps, which can develop high head pressures as well. The higher head pressure will generate even more flow, and hence, further improve heat transfer through the narrow flow passages of the tooling.
With no seals to replace and better, more precise heat transfer, water as a heat transfer fluid can be a better option for processors. The dollar savings can be substantial even before taking into consideration productivity gains from improved heat transfer, reduced floor space requirements, and a more environmentally friendly solution. Water is inexpensive, can be easily disposed of, and does not degrade when exposed to air.
Contact Information
Wittmann Inc., Torrington, CT
Jim Horne
(860) 496-9603
www.wittmann-ct.com
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