Analysis of critical radius of insulation for horizontal pipes

Thermal insulation of pipelines is used in different industries including agricultural technologies. Energy efficiency depends on heat losses, which, in turn, are limited by heat insulation. Decentralized ventilation is effective in rural low-rise buildings. One such device used is a regenerative ventilator like Blauberg Vento or Vents TwinFresh. There is a task to improve the efficiency of its operation. Investigation of heat recovery processes in thin ducts is taking place. Experimental uncertainty decreases with better heat insulation of the experimental setup. But in heat transfer literature, there is a notion of the critical radius of insulation. More the insulation radius should decrease its effectiveness due to the development of the outer surface. In the theory, the heat transfer coefficient of the outer surface is assumed to be constant. In this study, laminar convective flow around the outer horizontal cylindrical insulation surface of a large thickness is considered. The formula of the average heat transfer coefficient is used that takes into account the Grashof number. While the insulation thickness is increased, the surface temperature and the heat transfer coefficient are decreased. The function of thermal resistance dependence on the outer diameter related to the inner one and its derivative have been analysed. As a result, an asymptotic increase of thermal resistance was observed without extremums. So, there are no critical diameters if the insulation is so large to obtain laminar convective flow. The maximum radius of thermal insulation should be determined not as critical, but as expedient. Thus, it can provide 5–20% lower heat transfer resistance than the asymptotic one. The radius of thermal pipe insulation of technological devices should be accepted based on technical and economic justification.