Oil entrainment in vertical refrigerant pipingEntraînement de l''huile dans les tuyauteries frigorifiques verticales

The aim of this study is to investigate the required refrigerant speed, hence minimum refrigeration load, for carrying the lubricating oil up in vertical sections of refrigerant lines. It is assumed that the downward flow of the thin oil layer over the inner surface of the riser due to gravity is to be balanced with the upward flow of the oil film due to the shear force created by the upward flow of the refrigerant vapor. Velocities are converted to refrigeration capacities by considering a saturated cycle between specified condenser and evaporator pressures. General relationships thus developed are enumerated for R134a by preparing minimum capacity tables for copper suction and discharge risers.     

Evaluation of the column components size on the vapour enrichment and system performance in small power NH3–H2O absorption refrigeration machines

This paper presents an analysis of the influence of the distillation column components size on the vapour enrichment and system performance in small power NH₃–H₂O absorption machines with partial condensation. It is known that ammonia enrichment is required in this type of systems; otherwise water accumulates in the evaporator and strongly deteriorates the system performance and efficiency. The distillation column analysed consists of a stripping adiabatic section below the column feed point and an adiabatic rectifying packed section over it. The partial condensation of the vapour is produced at the top of the column by means of a heat integrated rectifier with the strong solution as coolant and a water cooled rectifier. Differential mathematical models based on mass and energy balances and heat and mass transfer equations have been developed for each one of the column sections and rectifiers, which allow defining their real dimensions. Results are shown for a given practical application. Specific geometric dimensions of the column components are considered. Different distillation column configurations are analysed by selecting and discarding the use of the possible components of the column and by changing their dimensions. The analysis and comparison of the different column arrangements has been based on the system COP and on the column dimensions.     

Numerical optimisation of a two-stage ejector refrigeration plant

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.     

The commercial feasibility of the use of water vapor as a refrigerant

This paper investigates the economic feasibility of a water-based vapor compression chiller with a nominal capacity of 3520 kW (1000 ton). Simplified models of potential cycle configurations are developed and used as a screening tool to identify a baseline cycle, the most attractive configuration for a water-based refrigeration machine. More detailed component-level models are developed to accurately size equipment and predict both the performance and cost of the baseline chiller. These component models address issues that are particularly crucial when water is used in refrigeration cycles, such as compression ratio, compressor discharge superheat and refrigerant-side pressure drop. Where possible, these component models are verified through comparison against the current state-of-the-art technology for large chillers that use R-134a as the refrigerant. The capital cost and expected operating costs are determined in order to quantify the payback and life-cycle costs associated with using water as a refrigerant, relative to traditional halocarbon refrigerants currently in use. Other issues that may have an economic impact on the feasibility of water as a viable alternative to traditional synthetic refrigerants are discussed, including purging and condensation within the compressor.The results show that water-based vapor compression refrigeration systems will not be economically attractive without substantial and successful efforts to develop low-cost, high capacity compressors. The paper provides an indication of the cost targets that must be met in order to make water vapor refrigeration systems practical.     

Condensate retention on a louver-fin-and-tube air cooling coil

This paper presents a study of condensate retention on a louver-fin-and-tube air cooling coil, which is commonly used in air conditioning (A/C) systems. Compared to previously related work focusing on the influence of condensate retention on the heat and mass transfer between air and a cooling coil, the present study emphasizes the impacts of operating parameters on condensate retention on a cooling coil. A new method to describe the steady-state condensation has been suggested and a new mathematical model to represent the force balance of retained condensate developed. The mass of condensate retained has been measured experimentally under various operating conditions of a direct expansion (DX) air cooling and dehumidification system. The influences of air dry-bulb temperature, moisture content and Reynolds Number on condensate retention are discussed. The model developed relates the mass of condensate retained to condensing rate, and is successful in predicting the trends of condensate retention under normal operating conditions for air cooling applications.     

Performance parameters for the design of a combined refrigeration and electrical power cogeneration system

This paper discusses the conservation of energy in a cogeneration system. A steam power cycle (Rankine) produces electrical power 2 MW and steam is bleeded off from the turbine at 7 bar to warm a factory or units of buildings during the winter or to supply a steam ejector refrigeration cycle to air-conditioning the same area during the summer. In the summer this system can be as alternative solution instead of absorption. Certainly the ejector refrigeration unit is more economical than absorption unit. The ratio of electrical power/heat is varied into the region (0.1–0.4) and the evaporator temperature of the ejector cycle is varied into the region (10–16 °C). A computer program has been developed for the study of performance parameters of the cogeneration system.     

Calculation of thermodynamic properties of R407C and R410A by the Martin–Hou equation of state — part I: theoretical development

A theoretical development of the thermodynamic properties of two mixtures of hydrofluorocarbon (HFC) refrigerants, i.e. R407C and R410A (in the superheated vapour state), is carried out. The modelling is based on the Martin-Hou equation of state, which has long been used for pure hydrofluorocarbons (e.g. R134a) with good results. Since R407C and R410A are very well investigated refrigerants, the analytical procedure here derived concerns with those thermodynamic properties of R407C and R410A (in the superheated state) that are not published in the current specialised literature. They are: compressibility factor, isentropic and isothermal compressibility, volume expansivity, isentropic and isothermal exponent, speed of sound and Joule–Thomson coefficient. These properties may be used as a theoretical basis for research into the optimal HFC-mixture for compressor efficiency and for performing cycle calculations in the vapour-phase region for systems working with R407C and R410A.     

Flow fields in shell-and-tube condensers: comparison of a pure refrigerant and a binary mixture

Detailed 2D CFD calculations for vapour flow field and rate of condensation are carried out for a geometry similar to a real shell-and-tube condenser with 100 tubes, with condensation on the shell-side. The differences in vapour flow behaviour are investigated for pure R22 and for a binary mixture of R32 and R134a, which has a gliding temperature difference of 5.5 K. It is shown that, the flow field for a zeotropic mixture is significantly different from that for a pure fluid. The nature of the mixture flow causes the vapour and condensate to flow counter-currently in part of the condenser. Adjustments of the inlet design turn out to influence the rate of heat transfer by up to 24% for the conditions tested, with greater influence on heat transfer for lower driving forces.     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.