There are two types of heat transfer fluid systems which are open and closed systems respectively. The expansion tank exhaust pipe of open system is connected to the atmosphere directly. While the exhaust pipe of closed system is insulated from the air.
The Open System
Advantages of open system are easy operation and good price performance as the direct connection of expansion tank exhaust pipe to the air not increasing the cost of system maintenance.
Fluid low boilers and moisture degrade into volatile gas and vapor in system operation, forming the liquid-gas mixed state. The mixed –state easily triggers system fluctuation under the system high temperature, high speed and low-pressure conditions, decreasing system effectiveness, reliability, safety and equipment service life. Especially when the temperature is over 300℃ issue shown above will aggravate greatly, causing fluid vaporization and even system break-off.
Fluid oxidizes in contact with the air and the oxidation speed is proportional to the contact area. Fluid large-area contacts with the air in open system and generates organic acid which accelerates polymerization reaction and produces impurities, increasing fluid viscosity and decreasing flow velocity, leading to worse heat transfer efficiency, shorter fluid service life and potential system safety risks.
The Closed System
Inert gas (nitrogen gas) or cold oil liquid seal devices are used to separate fluid from the air in the closed system. Fluid in closed system operates in enclosed environment which effectively prevents fluid oxidation and extends fluid service life. Compared to the open system, the closed system has lower risk in system oil injection and fire disaster.
Closed system sealed by nitrogen gas requires nitrogen gas source or the gas generation system, which complicates fluid system and increases the cost and complex of system operation and maintenance. System with nitrogen gas seal applies to large scale system or system equipped with nitrogen gas sources. Systems with no nitrogen gas source or limited budget usually apply to the cold oil liquid seal devices to separate fluid from the air.
It is suggested to adopt the closed system if the condition is up to one of the following requirements:
The system uses gas phase fluid.
The system fluid belongs to the dangerous chemical.
The system maximum operation temperature is higher than the fluid initial boiling point in usual atmosphere pressure. Or the fluid vapor pressure is higher than 0.01MPa when the system is under its highest operation temperature.
When the system fluid amount is over than 10m3
If the system heating demand and operation temperature change frequently.
