Rifled (grooved) tube air pre heaters - techno-commercial benefits

摘要

Plain tube air pre heater used for increasing the temperature of combustion air for boilers is defined as a indirect contact cross/counter flow heat exchanger. From the engineering point of view, any heat exchange should give the maximum heat transfer between the working fluids at the lowest cost. Classical heat transfer theory indicated that the preferred route of reducing heat exchanger cost was to increase the overall coefficient of heat transfer of the exchanger (symbol U Unit) (Kcal/M~2 - hr~0C). This logic followed from the governing equation of heat transfer Q = UALMTD where Q is the quantity of heat to be delivered. U - as explained, and A the exchange surface and LMTD the log mean temperature difference. Over the years many studies were carried out to determine means of enhancing U. Some were methodical, planned experiements with systematic tabulation and analysis of observations. Other findings arose due to observation of incidental practical phenomenon from various fields. For example,it was noted that cannons used for artillery firing, cooled quicker if they were grooved, as compared to non grooved gun barrels. This implied that grooving the barrels somehow improved (enhanced) the transfer of heat from within the barrel to the external surfaceand hence to the atmosphere. Systematic studies indicated that this was a result of several factors in combination: (a) The increasing of turbulence of the hot gases which positively enhanced the Reynolds number hence heat transfer.