At higher
temperatures, water ceases to become an effective heat transfer fluid. To
continue to use water as a heat transfer fluid it may become necessary to
pressurise the system, and substantial monitoring will be required to ensure
safe operation. Water can also cause corrosion within the system.
Mineral and
synthetic oils are suited for use as a heat transfer fluid at much higher
temperatures, and do not need to be pressurised until the top end of the range
is reached.
Although no
heat transfer oil is capable of meeting all of the below factors evenly, the
following factors should be considered, alongside the specifics of the
application:
- Low viscosity
- Good thermal stability
- High flash point
- Good heat transfer properties
- Ease of waste disposal
- Non-corrrosive
- Non-toxic
- Non-flammable
It is
important that the viscosity of heat transfer oils is low, especially when
operating at the lower end of the temperature range, as this will affect the
operating conditions under which the chiller is able to function. If the oil
becomes too viscous under lower temperatures, the system will not be able to
start up, causing damage to the chiller. The operating viscosity of the heat
transfer oil affects the flow properties within the pipes. The correct
viscosity, combined with the associate optimum turbulent flow, enhances heat
transmission.
The thermal
stability of heat transfer oils makes a large contribution towards pump
efficiency and the safe operation of the heat transfer system.
It is
important to ensure that high quality heat transfer oils are used. Heat
transfer oils are usually a mixture. It is essential that constituents with a
low boiling point are removed, as they will begin to evaporate during normal
operation of the chiller, their presence will reduce the viscosity and lower
the flash point of the oil. If the operating temperature of the chiller rises
with such constituents present, constituents with a much higher boiling point
are cracked, resulting in a high-viscosity substance with deposits a sticky
substance on pipes and surfaces.
When
selecting a heat transfer oil, it is important to look at the heat transmission
characteristics of the oil. The heat conductivity of the oil will give a good
indication of how well heat will transfer from the film coating the pipe walls
into the flowing heat transfer oil. The vapour pressure of the heat transfer
oil will indicate whether the system will be able to be run without
pressurisation. The thermal expansion-coefficient of the heat transfer oil will
indicate whether the oil is compatible with the size of the expansion tank.