29 Jun Comparison of Two Types of Heat Transfer Systems
There are two types of heat transfer fluid (HTF) systems – opened and closed systems. The opened system is that the expansion slot and exhaust pipes are connected directly to the atmosphere; while for the closed system the expansion slot is shielded from the atmosphere. The features and principles of two systems are presented below:
A. Features of opened system
The opened system is that the expansion slot and exhaust pipes are open directly to the atmosphere, which does not require equipment and maintenance fee and is easy to operate. Nearly 90% of heat transfer systems in China adopt this kind of system at present.
In this kind of system, low-boiling point volatile substance and moisture in HTF decompose into volatile gas and vapor in the process of heating, making HTF in a mixed state of liquid and gas. The working medium of mixed state can easily cause surge, pulsation and steam hammer shock under the conditions of high temperature, high speed and low pressure (in parts of the system), and it can also generate cavitation in high-pressure parts of system. Operating with surge and pulsation will not only decrease the efficiency, reliability and safety of equipment but also substantially shorten the service life. When the heating temperature is over 300℃, the saturated vapor pressure of HTF is greater than static head, the above-mentioned problems will be more obvious. What is worse, it can even make HTF vaporize rapidly, resulting in system breakdown.
Any type of oil will be oxidized in the air and HTF is no exception. The oxidation rate is in direct proportion to the contact area between fluid and oxygen. In the opened system, large area of HTF is constantly exposed to the air, generating organic acid, which accelerates the polymerization reaction to produce daub, leading to viscosity increment and velocity decrement of HTF. HTF’s increase in staying in furnace will result in lowering heat transfer efficiency, degrading HTF quality, shortening service life and impacting safety of system.
B. Features of closed system
The HTF is shielded from air in the closed system by sealing with inert gases (generally nitrogen is used) and cold oil. This kind of system can prevent HTF from oxidization and lengthen its service life. It can also decrease the volatilization of HTF, and avoid problems occurred in the opened system and oil blowout and fire hazard. The features of closed system completely comply with national call for energy conservation and emission reduction.
The nitrogen sealed system needs the source of nitrogen. Otherwise, a set of nitrogen gas generation system will be needed, which complicates the HTF system and raises the costs of operation and maintenance. Therefore, it is very difficult to popularize, and adopted only by major corporations, large-scale HTF systems and enterprises with nitrogen sources in China.
To businesses with no nitrogen sources, small-scale HTF systems and limited capital, cold oil is mostly used to separate HTF and the air, which can also prevent HTF from oxidation and lengthen its service life.
C. Principles of choosing opened or closed systems
The National Standard GB23971-2009 of Organic Heat Transfer Fluid and The National Technical Specifications on Safety of Special Equipment TSG G0001-2012 of Safety Monitoring Specification for Furnace have specified as to how to choose between the two types for HTF heating systems.
The National Standard GB23971-2009 specifies:
The L-QC and L-QD types (the highest allowed operating temperature is over 300℃) of HTF should be used in the closed system.
The L-QB type (the highest allowed operating temperature is under 300℃) of HTF can be used in both opened and closed systems. It regulates the condition of using HTF in opened system – the open flashing point and thermal oxidation stability should comply with the specifications and initial boiling point should be higher than the highest operating temperature.
According to Technical Specifications on Safety of Special Equipment TSG G0001-2012, systems meeting one of the conditions below should be designed as closed system.
1. Systems with gaseous phase HTF;
2. Systems with HTF that belongs to the dangerous chemical category;
3. Systems where the highest operating temperature is higher than the initial boiling point of selected HTF under ordinary pressure, or the vapor pressure of HTF is higher than 0.01MPa under the highest operating temperature;
4. Systems where the quantity of one-time HTF is over 10m3;
5. Systems with frequent changes of heating loads and operating temperatures.
Schultz has a lot of practical experience for both opened and closed systems, and is able to provide customers with economical, efficient, and customized equipment designs and operation techniques of liquid sealing.