Design, modelling and simulation of compact ammonia chiller and heat pump units

Large-scale ammonia refrigeration and heating systems have been widely used in industry due to their high efficiency, broad temperature ranges of operation and innocuous environmental effects. However, implementing this technology in small to medium systems has not been very successful. Toxicity of the refrigerant and ammonia equipment prices are obstacles this technology must overcome before entering a market dominated by systems running on HFC refrigerants. A comparison of ammonia properties with other refrigerants and a concise review of the research and development of ammonia systems is presented. After that, a preliminary design and modelling approach of a medium capacity low charge ammonia chiller and heat pump is presented. The correlations for heat transfer coefficient, pressure drop and void fraction that can be used for modelling the performance of plate heat exchangers are presented.

A 29 kW ammonia chiller with a highpressure receiver and direct expansion evaporator was used to collect data of different operating conditions. After that, the most representative data from the system were used to validate the numerical models of the plate heat exchangers. The model simulated the performance using several combinations of heat transfer coefficient correlations. The results showed that a suitable combination of correlations yielded maximum deviations of 4.8 % for heat transfer and 9.1 % for pressure drop in the condenser. Maximum deviations of 12.2 % for heat transfer and 57.3 % for pressure drop were calculated for the evaporator. Furthermore, an estimation of refrigerant charge distribution showed that the evaporator would tend to accumulate the largest share of refrigerant with a maximum of 1 kg when the chiller was working at 725 RPM.

A 31 kW ammonia chiller with a split condensation process and direct expansion evaporator was used to collect data of the system when used with different amounts of refrigerant charge. The results showed that a suitable combination of correlations yielded maximum deviations of 1.9 % for heat transfer and 8.8 % for pressure drop in the condenser. Maximum deviations of 12.7 % for heat transfer and 8.3 % for pressure drop were calculated for the evaporator.

A design of two medium capacity low charge ammonia chiller and heat pump units was proposed based on two different design conditions. The units have a low pressure receiver, a direct expansion evaporator, and a subcooling control approach. The plate heat exchanger models for condenser and evaporator with the identified correlations for heat transfer coefficient, pressure drop and void fraction were used to design the optimum geometry of plate heat exchangers that could guarantee low refrigerant charge.

A dynamic model of the chiller and heat pump units was implemented on Dymola using the identified correlations for heat transfer coefficient, pressure drop and void fraction. The models helped to study the control structure of the units when shifting between the different modes of operation. As a result, the dynamic simulations showed that the subcooling control approach would only work when control values below 10 K are used when shifting between the different operating modes. Otherwise, the system controller would saturate due to the lower values of effective flow area needed by the electronic expansion valve when selecting higher values of subcooling.