Heat transfer and pressure drop at refrigerant boiling in the plate coils
Boiling of refrigerant in pipes requires thermal and hydro-mechanical calculations for the evaporators. Under condition of transition to the new refrigerants generalized correlation for saturated two-phase flow boiling heat transfer coefficient (HTC) and pressure drop inside horizontal tubes, which uses known conditions of boiling and quantitative data of the refrigerant properties, are of great particle interest. Five generalized methods for calculating the local HTC and four methods for calculating pressure gradients of the boiling liquid in horizontal pipes were selected. A method and a program for joint calculation of HTC and pressure drop during boiling of ten refrigerants in the coils with the number of horizontal pipes from 2 to 20 were made. The comparison of the calculated HTC average values in a tube downstream and pressure gradients with the available experimental data were made. Examples of the impact of structural and operational parameters on the HTC, pressure drop, and heat influx of coils are given. The pressure drop and HTC in coils are shown to affect the heat flux density and the relative length of the pipe most significantly, which at a selected pipe diameter determine refrigerant mass velocity and heat influx of the coil. The program made can be used for the coil parameter selection to provide highest HTC with the allowable pressure drop for a particular refrigerant.