Thermal liquid heating is a specialized form of process heating that utilizes the forced circulation of special heating medium as a liquid. In many types of process heating, high temperature, rather than high pressure, is essential where heat, not vapor or steam per se, is required. Thermal Liquid Systems can generally be used at high temperature without a corresponding high pressure.

System Components

Heater
The Parker Direct Fired Type Heater represents an ideal heater for thermal liquids. Our all welded, bent steel tube design allows for the continuous expansion and contraction to which the heater must be subjected without damage. Please refer to our Brochure 207 for internal construction details. The double welded construction eliminates the problems of rolled or poor joining of tubes, experienced in competitive units. This type of heater is preferred by more Fluid Manufacturers. Thermal liquids at advanced temperatures are so thin that only the finest welding can contain the fluid without leakage. Since all fluids will burn in the ambient temperature of the gas flame, this leakage can be serious with other than our design. The effects of thermal liquid shock are nullified in our serpentine design tube assemblies.

Fluid
The fluid to be used must be selected from a wide variety of specifications. Fire resistance, cold flow and freeze points, high operating temperatures, viscosity at varying temperatures, life expectancy, vapor pressure characteristics, and of course, fluid cost are among the deciding factors in fluid selection.

Pumps
In closed recirculating Hot Water Systems, pump selection in GPM is based on the desired temperature rise through the heater and the system head pressure. Pump selection in a thermal liquid system is based on velocity flow across the tube surface and the total system head. Some fluids reduce viscosity by 100 times from 50 centistokes at 100° F to .5 centistokes at 600° F. Velocity flow should not be less than 8 feet per second, which with 1" pipe as boiler tubing represents approximately 20 GPM per tube. Due to the change in viscosity with change in temperature, pump motor horsepower must be selected by considering the cold point and hot point of the system. Parker can assist in selection and provide air cooled or water cooled pumps.

Tanks
Selection of the system expansion tank can generally be based on the simple formula of 4% expansion per 100° F of temperature. This figure will be accurate enough for most applications. The tank should be sized to be 1/4 full at 70° F and 3/4 full at maximum system temperatures. A sight glass and low liquid level cut-off should be installed on the tank to monitor liquid level. The tank should be mounted at the highest point in the system. Provisions should be made to vent the system to the correct opening in the expansion tank. Refer to piping diagrams. Expansion Tank pressurization with nitrogen or air may be necessary.

System Arrangement/Specification
A central system distribution tank for primary secondary pumping systems is common and recommended when flow thru the heater cannot be guaranteed with a one pump system. All system piping over 1" should be welded. When flanges are required, a high grade Flexatalic high temperature spiral wound gasket must be used. No petroleum type pipe dope should be used. All long circulating lines should be protected with expansion joints to prevent damage due to expansion.