Heater Start-Up and Shutdown Procedures

Start-Up and Shutdown Procedures

Regardless of system design, size or heat source, a few basic precautions and procedures must be followed during start-up and shut-down of heat-transfer fluid systems. The following procedures will help you to ensure maximize the service life of heat-transfer fluid.

System Start-Up

A heat-transfer fluid at room temperature may have a viscosity as high as 30 cSt but if the system is outdoor and if the ambient temperature is below 0 °C the viscosity could even be as high as 100 cSt or higher.

While a fluid with such high viscosity can be handled by the pump and circulated in the system, it would not be ‘ready’ to absorb the full heat efficiently due to low convection. Any heater, small or large, is designed to transfer heat at a predetermined rate in conjunction with the flow rate of the fluid. When a fluid hasn’t achieved the ideal viscosity and consequently its operating flow rate, it will not match the rate of heat transfer. In other words, if the fluid is too viscous it will move too slow past the heat source and absorb too much heat, too quickly. Therefore, if a heater is allowed to fire up at full capacity during start-up, it will most likely overheat the fluid locally and result in thermal degradation of the fluid.

Therefore at the time of system start-up it is important to allow for gradual increase in temperature until fluid flow or velocity is reaches the range specified by the operating parameters. As a precaution you must ensure that the pumps are running at set parameters. Generally a 20 °C to 30 °C incremental increase in the set-point will allow for steady and even heating without the chance of overheating or cause thermal degradation of the heat-transfer fluid.

If there is any moisture ingress in the system it would be indicated by vapour being vented out of the vent line in the expansion tank and can also cause fluctuations in the system pressure. Whenever these conditions are observed, heating must be stopped to maintain the same temperature until the vapour is vented out completely. Once the venting is complete and the pressure fluctuation has stopped, heating can then be resumed from the same temperature point.

System Shutdown

During heater shutdown, a few basic steps will help to ensure that no damage from overheating is inflicted on the heat-transfer fluid.

During the course of normal operation, irrespective of its size, the heater will be cycling either on and off or from a low fire to a high fire in order to maintain the set temperature. At any given time of firing, the heater piping or vessel may become nearly as hot as the heat source itself. It is also important to remember that the actual temperature of heat transfer surfaces of piping inside the heater in most cases may even be higher than that of the recommended maximum fluid temperature for the heat-transfer fluid.

If a system is shutdown abruptly without allowing the heat source and adjacent areas to cool when the fluid ceases to flow it will become trapped and subsequently ‘burn’ or experience thermal degradation. Therefore when shutting down any system it is important to simply allow the heat-transfer fluid to cool below 250 °F (121 °C) before shutting down the pump. The use of a heat-exchanger or allowing the heater’s blower to continue to run will help to expedite lowering of the heat-transfer fluid temperature to under 250 °F (121 °C).

Leave a comment

Your email address will not be published. Required fields are marked *