Shell Side Direct Expansion Evaporation of Ammonia on Plain Tube Bundle with Inlet Quality and Exit Superheat Effects

Abstract

Shell and tube heat exchangers are widely used for various industrial refrigeration applications. There is no data available in open literature on the direct expansion evaporation characteristics of refrigerants on the shell side of tube bundle. With the current Ozone Depletion and Global Warming issues it is critical to develop systems with low charge refrigerants especially with natural refrigerants such as ammonia which has zero Ozone Depletion Potential (ODP) and zero Global Warming Potential (GWP). A comprehensive literature survey has been presented to summarize previous single phase correlations for estimating heat transfer coefficient inside circular tubes and studies performed with ammonia boiling outside single tube and bundles and the correlations thereof developed. Considerable amount of work has been performed and correlations developed for estimating heat transfer and pressure drop in smooth circular tubes for single phase flow. There are limited studies available on pool boiling and spray evaporation of ammonia outside single tube and bundle. As of now, there are no studies available in the open literature on the subject of direct expansion evaporation of refrigerants. The basic purpose of the present study is to quantify the effects of various parameters such as saturation temperature, heat flux, inlet quality and degree of exit superheat on the heat transfer performance for ammonia boiling on the shell side of tube bundle under direct expansion mode, and to develop empirical correlations. Overall performance of the heat exchanger and performance of individual passes are also studied. The design and details of the single phase and two phase experimental setup, instrumentations, data acquisition system and test heat exchanger are described. Single phase experiments were performed to develop single phase correlations for specific geometry and flow conditions for estimating heat transfer coefficient inside tube bundle using simple Wilson plot and modified Wilson plot method. Prandtl number was varied from 10 to 20, with Reynolds number ranging from 3000 to 15000. The correlations developed are compared vii with existing single phase correlations and were found to be in good agreement with the previous studies. Two phase experiments were performed at steady state conditions with ammonia boiling on the shell side of heat exchanger, while water/glycol solution flowing in the tubes. Saturation temperature was varied from -1.7 to -20 , inlet quality was varied from 0 to 0.3, and heat flux was varied between 5 and 45 , with exit degree of superheat ranging from 2 to 10 . Heat transfer coefficient was observed to increase with saturation temperature and heat flux and decrease with exit superheat (quality) because of the diminishing effect of boiling and dominance of single-phase sensible regime. The effects of inlet quality on the average bundle performance were found to be negligible. Heat transfer coefficient slightly decreased or remained unchanged with inlet quality. The quality effects were found to be more dominant at higher saturation temperature than at lower saturation temperature. Bundle effect was observed between the passes which was a function of saturation temperature, heat flux and exit degree superheat. The present data suggests that bundle effect is less significant at low heat flux, with lower degree of exit superheat. The variations in heat transfer coefficients of individual passes are usually smaller under these circumstances. At higher exit degree superheat and heat flux the upper tubes experienced greater dry out which resulted in a sharp drop in their performance. The trend was similar across the saturation temperature range. The effects of exit degree of superheat became more dominant on the bundle performance over heat flux when ammonia was subjected to vapor quality at the inlet of the evaporator. With increase in the inlet quality the performance of lower three passes converge. Two phase pressure drop under direct expansion mode was found to be sufficiently small. The effects of saturation temperature and inlet quality on pressure drop were negligible. Pressure drop marginally increased with decrease in the degree of exit superheat and with increase in the heat flux.