Conference Year

2016

Keywords

Low Lift reversible Heat Pump, Freeze Pre-Concentration System, Thermal Mass, COP, Inclusion

Abstract

Freeze pre-concentration system, FPCS, with low lift reversible heat pump is designed for selective freezing of water from sugarcane juice. Two identical vented double wall tube-and-tube heat exchangers are used, to freeze water in the evaporator and melt ice in the condenser, alternately. They are sized based on cooling capacity offered by compressor, ice growth rate and velocity of juice inside the tube to reduce inclusion. Low lift heat pump is designed to operate at -8oC evaporator and 3oC condenser saturation temperatures. Since, the condenser duty is higher than evaporator duty excess heat duty, in the form of superheat is utilized to heat pre-concentrated juice. Raw juice is precooled in a three stream Tube-and-Tube Heat Exchanger, TT_HE. Raw juice is precooled using cool concentrated juice and separated water.  Freeze pre-concentration of sugarcane juice from 20°Brix to 40°Brix using a low lift reversible heat pump saves bagasse during initial 63% water removal. Water is removed through the freezing process requires 335 kJ/kg heat removal, which is equivalent to 15% of heat addition during evaporation at atmospheric pressure in open pan in jaggery making.  Investigations on selection of refrigerant R744, R290 and R22 for FPCS are presented.  R290 is identified as preferred refrigerant. It is natural refrigerant, no ODP and significantly lower operating pressures compared to R744. R290 charge of 360 g for 1.5 TR compressor based system is managed by using small diameter refrigerant side tubes. It address safety related issues for modular small capacity systems.  Superheat temperature of compressed refrigerant is 9oC for R290. It allows to size the identical LHEs with R290 as a refrigerant. Flashing of refrigerant in evaporator is 7% for R290 and 10% R744. Generally, R744 is preferred when high temperature heating is required. But, high superheat at compressor outlet and increased flashing at evaporator inlet reduces the performance of R744 system making it less preferred as compared to R290 and R22. Theoretical cycle COPc calculated for R290 based reversible heat pump works out to be 20, with compressor isentropic efficiency of 70%. Overall system COPc is in the range of 10 to 13 after accounting for losses like cycling of thermal mass, heat gain from ambient, variation in freezing point depression and ice layer thickness. System Carnot efficiency is in the range of 41 to 54%. Power required for 1.5 TR FPCS is 0.4 to 0.6 kWe.  Different juice side tube diameters are considered to find the optimal size, after accounting for effect of thermal mass of heat exchanger, heat pump switching time and inclusion on the energy consumption per unit water separated.  Achievable energy consumption is in the range of 9 to 12 kWhe/m3 of water separated.

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