® Solutia Inc. P.O. Box 66760 St. Louis, Missouri 63166-6760 (314) 674-1000
Pub. No. 7239124B (Supersedes 7239124A)
HEAT TRANSFER SYSTEM EXPANSION TANK DESIGN
The design of the expansion tank in a liquid phase heat transfer system using Therminol® heat transfer fluid or other organic fluids is an important parameter in the total system’ssuccessful operation. An expansion tank correctly installed and maintained can contribute to increased fluid life along with lower maintenance associated with the various mechanical components of the system, such as pumps, gaskets, seals and heaters. A properly designed expansion tank can eliminate many problems from the initial startup through everyday operation of the heat transfer system. Thefollowing discussion will explore generally the purpose and design of an expansion tank in a heat transfer system. However, a qualified engineering firm should be consulted in connection with the design of an actual heat transfer system, since considerations outside the scope of this bulletin may be critical. Figure 1
Purpose of the Expansion Tank in a Heat Transfer System
As the term implies,the main function of the expansion tank in a heat transfer system is to provide for fluid expansion, which can be greater than 25% of its original volume depending on the fluid used and the operating temperature. Since the tank is usually installed at the highest point in the system, it also can serve as the main venting point of the system for excess levels of low boilers and moisture which mayaccumulate in the heat transfer fluid. The highest point installation also creates positive head pressure to the pump’s inlet, providing flooded pump suction with uninterrupted flow of fluid to the user station. A simplified drawing showing a suggested positioning of the expansion tank in a heat transfer system is labeled Figure 1.
The Basic System*
*NOTE: This information does not constitutean express or implied warranty. See NOTICE on the back of this bulletin.
Expansion Tank Design Parameters
There are several basic design parameters which should be considered part of every heat transfer system’s expansion tank so that the maximum benefit can be obtained from the tank relative to overall system operation. Sizing The expansion tank should be sized so that it is 25% full atambient temperature and 75% full at normal operating temperature. This sizing should cause positive fluid pressure to the pump’s suction side during system startup and should minimize the vapor space in the tank during normal operation. Fluid expansion between two temperatures can be calculated by dividing the fluid’s density at the lower temperature by the density of the fluid at the highertemperature — i.e., the density of Therminol® 66 at 40˚F is 8.47 lb./gal. and changes to 6.72 lb./gal. at 600˚F. Thus the expansion of Therminol 66 is 8.47/6.72 = 126% of the original volume at 40˚F when heated to 600˚F. Therefore, an expansion tank for a 1,000-gallon Therminol 66 system operating between 40˚F and 600˚F should be sized for 260 gallons of expansion. Since this expansion represents 50% of thetank volume (the volume between 25% and 75% full), the expansion tank should be 520 gallons in size. Double Drop Leg and Valving The expansion tank should be located at the highest point in the system with a double drop leg piping arrangement as is shown in Figure 2. If the only purpose of the expansion tank was to provide for fluid expansion, a single drop leg would be sufficient; however, theexpansion tank also provides the best point for system venting. In order to properly vent a heat transfer fluid system, the expansion tank must be capable of full system flow. The lines to and from the expansion tank should be sized to take full system flow at startup conditions.
Under normal operating conditions, valves A and C are open, and valves B, D, E and F are closed. To add make-up...