MAXIMUM THERMAL EFFICIENCY OF CONDENSING BOILERS USED FOR A HEATING SYSTEM

Since 1970s, all kinds of condensing boilers (CB) have been widely applied in HVAC engineering, their exit flue gas temperature is very lower, even down to about,40 degrees C-45 degrees C and their thermal efficiency is very higher, even up to about 107%similar to 109%, based on low heat value. The application of CBs in HVAC system or hot water supply system can increase thermal efficiency and reduce the consumption of fuel, because of the utilization of latent heat of vapour in the exit flue gas. Generally, the higher the content of vapour in the exit flue gas and the lower the temperature of return or make-up water, then the lower the: exit flue gas temperature of CB and the higher the thermal efficiency of CBs. And then what is the maximum value of thermal efficiency of CBs theoretically? In this paper, from the thermal equilibrium and the definition of thermal efficiency based on low heat value (net heat value), the maximum value of thermal efficiency of condensing boilers that burn different typical fuels have been educed, which are respectively as follows: 107.1%(0 # diesel oil), 110.4%(natural gas), 105.2%(bituminous coal) etc, which are useful to both the design of condensing boilers in heating system and the design of HVAC engineering.