Investigation of Parallel Flow Heat Exchanger Response to Step Increase in its Heat Load

Akram W. Ezzat; Salah Sabeeh Abed AlKareem; Ahmed Salam Abdul-Raheem

Akram W. Ezzat Department of Mechanical Engineering / University of Baghdad
Salah Sabeeh Abed AlKareem Department of Machines and Agriculture Equipment Engineering / Machines and Agriculture Equipment Engineering
Ahmed Salam Abdul-Raheem Department of Mechanical Engineering / University of Baghdad

Keywords: open circuit heat exchanger, heat load transient, parallel flow, response time

Abstract

An experimental and theoretical analysis was conducted for simulation of open circuit parallel flow heat exchanger response to power increase transient in its heat load. Proper mathematical model is prepared to cover the heat transfer mechanism between the hot water in the primary circuit and the cold water in the secondary circuit during transient course. The model takes under consideration the effect of water heat up in the primary circuit due to step increase of its heat load added to same circuit on the physical and thermal properties linked to the parameters that are used for calculation of heat transfer coefficients on both sides of their tubes. Proper Math-Lab computer program was prepared for calculation the steady and transient states. The calculations covered all the variables that affect such type of transient mechanisms. The nominal power of the heat exchanger of 618w is subjected to step power increase transient of 880 w. The effect of the power increase percentage in the primary circuit on the average temperature buildup of the water in same circuit was investigated. The results covered the effect of power increase percentage of 130% of its nominal value during steady state for different secondary flow rates (120liter/hr, 170liter/hr and 200 liter/hr). The elapsed time required for the primary circuit average temperature to reach a steady state value was also calculated for different primary and secondary water flow rates. These calculations were supported with experimental measurements conducted on a standard parallel flow heat exchanger apparatus. The experimental results were compared with the theoretical results at fixed primary and secondary flow rates which showed reliable agreement with a maximum deviation of 4.2%. The results proved that water average temperature build up in the primary circuit has sharp tendency during the early stage of the transient course and then it reached to its saturation value after around 8-10 minutes.