Regardless of system design, size or heat source, a few basic procedures should be followed when starting up or shutting down a heat transfer system. Duratherm Extended Life Fluids, Lewiston, N.Y., says following these basic procedures can help maximize the life of fluids.
During system startup, keep the following pointers in mind:
- A fluid at room temperature may have a
viscosity as high as 100 Cst, but if the system is outdoors and the ambient
temperature is below 32°F (0°C), the viscosity could be 1,000 Cst or higher. A
fluid at these viscosities is easily pumpable but is not yet ready for full
- A heater, whether small and electric or large and
gas fired, is designed to apply heat at a set rate in consideration of the
fluids flow or velocity. When a fluid has not achieved the ideal viscosity, its
flow or velocity will not be of that specified and required by the heat source,
meaning the fluid will be too thick to allow efficient flow.
- If a heater is allowed to fully fire during these
periods, it likely will overheat and thermally degrade, meaning the fluid will
pass the heater slowly and absorb too much heat.
- When starting up any system, it is important to
allow gradual temperature increases until the fluid’s flow or velocity is
within the boiler’s required range.
- Generally, a 20 to 30°F (11 to 16°C) incremental
increase in the setpoint will allow for steady, even heating without the chance
of overheating or thermally degrading the fluid.
- During the course of normal operation,
both small electric heaters and large gas-fired boilers will be cycling either
on and off or from low fire to high fire to maintain the set temperature.
Within a short period of time, the heater piping or vessel will become nearly
as hot as the heat source itself.
- It is important to remember that the heater is
actually hotter than the output temperature. The actual temperature at the
impingement point of the heater may be higher than that of the recommended
maximum fluid temperature.
- Avoid abruptly shutting down the system without
allowing the heat source and adjacent areas to cool. If you don’t, when the
fluid ceases to flow, it will become trapped and subsequently burn or thermally
- When shutting down any system, it is important to
allow the fluid to cool below 250°F (121°C) before shutting down the pump.
- Using a heat exchanger or leaving the heater
blower running will help expedite cooling the fluid temperature to below 250°F.