Abstract
An air-to-air heat pipe heat exchanger has been designed, constructed and tested. Gravity-assisted wickless heat pipes (thermosiphons) were used to transfer heat from one air stream to another air stream, with a low temperature difference. A thermosiphon heat exchanger has its evaporation zone below the condensation zone. Heat pipes allow keeping a more uniform temperature in the heat transfer area. The heat exchanger consists of 20 copper tubes with circular copper fins on their outer surface. The tubes were arranged in a row and the air passed across the pipes. R245fa was used as a working fluid in the thermosiphons. Each heat pipe had a 40 cm evaporation section, a 20 cm adiabatic section and a 40 cm condensation section. The thermosiphon heat exchanger has been tested in different conditions of air stream parameters (flows, temperatures and humidity). The air face velocity ranged from 1,0 m/s to 4,0 m/s. The maximum thermal efficiency of the thermosiphon heat exchanger was between 26÷40%, depending on the air velocity. The freezing of moisture from indoor air was observed when the cold air temperature was below - 13°C.
Highlights
Heat pipe heat exchangers (HPHEs) are used as an efficient air-to-air heat recovery device in many applications
The HPHE provides the separation of hot and cold flows which is essential for heat recovery systems in hospitals, laboratories and other places where a high quality of indoor air is needed [1]
In this paper we focused on the two-phase closed thermosiphon (TPCT) which is a heat pipe without the wick structure inside the pipe’s inner
Summary
Heat pipe heat exchangers (HPHEs) are used as an efficient air-to-air heat recovery device in many applications. In HPHEs, different kinds of pipes could be used: conventional heat pipe (CHP), two-phase closed thermosiphon (TPCT) and oscillating heat pipe (OHP). The aim of this work is to rate the use of HPHE in the heat recovery of exhaust heat from an air conditioning system utilizing gravity assisted two-phase closed thermosiphons. In an HVAC system heat is recovered from exhaust air, the temperature of which rarely exceeds 20oC and the fresh air temperature is often below 0oC In such conditions water, but some other working fluids could be taken into account, especially refrigerants. CHPs and TPCTs water is used as a working fluid in heat pipes [6, 7], but these solutions are proper for temperatures higher than 40oC. The saturation pressure of refrigerants is higher than the saturation pressure of water, which causes a higher heat transfer coefficient during the phase change inside the pipe
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