When designing liquid-phase heat transfer systems, much of the focus is on the furnace, heat exchangers and pumps. But, the expansion tank is a critical component that should not be overlooked.
Every liquid-phase heat transfer system needs an expansion tank to allow for thermal expansion of the heat transfer fluid during normal operations. While this is the primary function of the expansion tank, it serves a number of other functions as well. These include:
- Separating low boilers and moisture from the system.
- Providing pump suction head pressure for the circulating pumps.
- In some cases, acting as a storage tank for the system volume during maintenance.
Expansion Tank 101: A Glossary of Terms
Bulk Temperature — The temperature of the fluid in the system.
Dead Band — The range of a controller that allows for minor fluctuations without opening or closing a valve.
Inert Blanket — A non-oxygen-containing gas used to pressurize the space above the liquid in the expansion tank.
Low Boilers — Low molecular-weight compounds formed when a heat transfer fluid thermally degrades.
Maximum Operating Temperature — The highest temperature at which the heat transfer fluid will be heated.
Noncondensables — Compounds that remain as a gas at normal temperatures.
SCFM — Standard cubic feet per minute.
Vapor Pressure — The pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.
This article will discuss sizing an expansion tank, piping and instrument setup, and recommended operating procedures for liquid-phase heat transfer systems. While they highlight many important factors to keep in mind, these guidelines are not all-inclusive. Qualified design engineers should be consulted for a complete system design.
Expansion Tank Sizing
Expansion tanks should be sized to accommodate the maximum expected expansion of the heat transfer fluid being used in the system. This can be calculated with the following equation:
ρ(TLOW ) – ρ(THIGH ) x System Volume = Expansion Volume
ρ(TLOW) is the density at the lowest anticipated startup temperature.
ρ(THIGH) is the density at the highest anticipated temperature.
Density data should be available from your fluid supplier. Over its use range, typical expansion of a synthetic organic heat transfer fluid is 25 to 50 percent. In the case of silicone heat transfer fluids, it can be as high as 70 percent.
Now that the expansion volume has been calculated, this value can be used to calculate the total size of the tank required. A typical tank should never be operated at less than 15 percent full and not more than 80 percent full. This prevents entrainment of vapor at cold startup and minimizes the possibility of overflowing the tank at high temperature. As a result, the tank size can be calculated from:
(Expansion Volume) / 0.65 = Tank Size
An alternative method of sizing the tank is to determine the volume of the system and specify a tank large enough to accommodate the entire fluid volume at maximum temperature. This method allows the system to be drained to the expansion tank for maintenance, but it also results in a much larger tank.
Location and Piping
The expansion tank typically is located at the high point in the system to allow optimum venting. Water, air and low boiling decomposition products will naturally rise to the high point in the system because their vapor pressure is higher and vapor density is lower than the vapor pressure and vapor density of most synthetic organic heat transfer fluids.
Locating the expansion tank at the high point in the system also maximizes pump suction pressure to prevent pump cavitation. The net positive suction head (NPSH) requirements of the pump must be considered when locating and determining the pad gas pressure needed on the expansion tank.
If site considerations do not allow the expansion tank to be located at the high point of the system, it must be kept under sufficient pressure to keep fluid at the high point of the system. The tank and system also must be designed to handle the additional pressure. The pressure rating for the tank should take into account the vapor pressure of the fluid at its maximum operating temperature. If a fluid is to be operated as a liquid above its boiling point, it is recommended that the expansion tank pressure be maintained at least 5°F (3°C) above the vapor pressure of the fluid at the highest bulk temperature in the system. Normally, this will be the bulk fluid temperature at the outlet from the heat source. This will prevent the fluid from flashing to vapor in the piping, causing increased pressure drop and reduced flow in the system.
For ease of startup and periodic venting of the system, the expansion tank should be designed to allow full flow of liquid through the tank. A double-drop-leg design (figure 1) is the most effective arrangement to remove air, water vapor and other noncondensables during system startup. The tank should be piped so that the inlet and return legs are both below the level of the liquid in the tank. For subsequent operation, the flow through the tank can be reduced. Periodic venting of low boiling degradation products may be necessary to maintain proper pump operation.
Piping and a pressure regulator for an inert, non-oxygen-containing blanket should be provided. The most common gas used is nitrogen, but natural gas also is used in the oil-and-gas industry. The important thing is to keep out air and moisture. Air will cause fluid oxidation while water can cause excessive venting and system overpressure along with potential corrosion in the headspace of the tank. The inert gas blanket should allow for a continuous flow of inert gas to be purged through the vapor space of the tank during startup. Separate inert gas supply and discharge vent nozzles — spaced as far apart as possible on the top of the tank — will help ensure that any volatiles such as water or solvents will be swept from the system during initial startup.
The discharge vent also should be equipped with a backpressure regulator to keep the desired pressure on the tank. It should be noted that a dead band could be desirable between the supply gas pressure and the backpressure setting to minimize gas consumption, especially for systems where the temperature fluctuates a lot.
The expansion tank also should include a safety-relief valve. Both the backpressure vent line and the safety-relief valve should be piped so that they discharge to a safe area away from potential sources of ignition and areas where operating personnel may be affected by a release. For fluid with a high freezing point, care should be taken to trace headspaces and vent lines to keep them above the fluid’s freezing point at all times. Proper relief design is important and should only be done by qualified personnel.
Normally, additional instrumentation — a level indicator, fluid-temperature indicator, both high and low level alarms, and a pressure indicator with a high pressure alarm — is added.
System Operating Procedures for Synthetic Organic Heat Transfer Fluid Systems
As noted, the expansion tank is important during system startup. In most new systems, there will be some residual water due to pressure testing or water infiltration during construction. A new system also normally contains a lot of trapped air. To remove these and other contaminants, the fluid should be circulated through the expansion tank and a flow of 1 to 2 scfm inert gas established through the tank’s headspace. As the system is heated slowly, the contaminants will separate from the liquid and be removed through the vent system. Heating at a rate of 50°F (30°C) per hour and holding at each increment until all water has vented should ensure that no water is left in the system once 350°F (177°C) is reached. At this time, venting can cease, and the system can be allowed to heat up to operating temperature. The backpressure regulator should be adjusted to the desired tank pressure at this time.
Once the system is operating, it may be necessary to vent low boiling decomposition products periodically. Failure to vent low boiling degradation products can result in increased fluid degradation, pump cavitation, fluid flashing in the furnace and system overpressure. For optimal venting of low boilers, it is important to have some control over the flow rate of the fluid circulating through the expansion tank. If the system is operating at high temperature or above the boiling point of the fluid, venting with full flow through the expansion tank and the tank at operating temperature can cause excessive loss of good fluid while also removing the low boilers.
For optimum low boiler venting, the tank temperature should be maintained just above the boiling point of the low boiling degradation products of the fluid being used. The fluid manufacturer should be able to provide information regarding the boiling points of common degradation products for their fluid. Maintaining this temperature while sweeping the headspace with inert gas as described in the startup information earlier in this article will provide good removal of low boilers without excessive loss of good fluid. Periodic fluid analysis should be used to determine the frequency and duration of required low boiler venting.
This information should help to explain the importance of the expansion tank in a heat transfer system. Proper sizing and design — combined with inert blanketing and periodic venting — can help ensure many years of trouble-free operation of your liquid-phase heat transfer system.