Industrial refrigerating machines

The following exercises are designed to familiarize the reader with calculations for the essential components of refrigeration circuits: compressors and exchangers. They do not take into account essential accessories such as safety devices, automaticity and control. The selection of these devices is based on suppliers' catalogs, and requires no precise calculations.

In recent years, the refrigeration industry has suffered a major shock following the eradication of a number of refrigerants, either strictly banned by the Montreal Protocol (CFC chlorofluorocarbons), or in the process of being phased out under the Kyoto Protocol (HCFC hydrochlorofluorocarbons). Today, professionals have chemical substitutes at their disposal: hydrofluorocarbons HFCs, but these do not appear to be sustainable due to their often very high GWP (Global Warming Potential). Faced with this situation, there has been renewed interest in so-called natural fluids, including ammonia, hydrocarbons and carbon dioxide.

One of the most widely used HCFCs today is R 22, no doubt because of its thermodynamic properties, which are similar to those of ammonia, but without its shortcomings. The timetable for its eradication sets 2010 ~ 2015 as the target date for a total ban.

One way of extending the life of an existing R 22 system is to replace it with a more durable product. This is the subject of Exercise 1, which deals with changing the fluid in a water chiller.

Exercise 2 involves the calculation of a new water-cooling unit, based on the use of a zeotropic HFC with high temperature drift, R 407C. This refrigerant is considered, along with R 410A, to be the preferred choice. However, these products have high GWPs, respectively 1,600 and 1,900, compared with R 22: 1,700.

The annual use of a water chiller unit is subject to various fluctuations in climatic and occupancy conditions in the building served by the unit, which affect the condensing and evaporating temperatures. A simple methodology, described in Exercise 3, can be used to predict the effect of these fluctuations on the performance of the unit calculated above, and to estimate its annual energy consumption.

Exercise 4 describes methods for calculating refrigeration balances, either for a cold store (daily energy balance) or for a rapid refrigeration or freezing system (dynamic balance).

This leads to Exercise 5: design of a conventional industrial refrigeration circuit using ammonia.

Exercise 6 compares a low-temperature freeze-drying plant with a single-fluid refrigeration circuit and a "cascade" circuit using carbon dioxide in the first stage and ammonia for condensation. Comparison of the energy...