A brief selection procedure of suitable refrigerant pairs in cascade refrigeration system along with selection charts based on various performance criteria are presented, which will be very useful for any design engineer to select suitable natural refrigerant pair.
Cascade Refrigeration System
Many industrial and medical applications require ultra-low-temperature cooling, which cannot be achieved effectively by single-stage or multistage systems due to individual limitations of the refrigerant, hence, a cascade system is the best alternative in these situations. Suitable selection of refrigerants used in high-temperature (HT) and low-temperature (LT) cycles – can provide the required low temperature while attaining good system efficiency. For cascade systems, the lower temperature limit of the HT side is termed as the intermediate temperature (IT); the system performance depends upon IT and hence, it needs to be optimised to obtain maximum performance. A schematic diagram of a two-stage cascade refrigeration system is illustrated in Figure 1.
The HT and LT circuits are thermally connected to each other through a cascade-condenser, which acts as an evaporator for the HT cycle and a condenser for the LT cycle. In practice, there is a certain overlap between IT and condenser temperature of LT circuit and the difference between these two temperatures is called the Overlapping Temperature (OT).
Synthetic refrigerants are being phased out worldwide to combat the twin menace of the ozone layer depletion and global warming. Attainment of holistic environmental safety is the call of the day, and this has facilitated the recent emergence of natural refrigerants as the more benign working fluid in refrigeration and heating systems. Several natural refrigerants are regaining their importance and are on a path of revival.
Hence, the cascaded systems based on natural refrigerants are offering good potential for research in these circumstances for HT lift. However, for a refrigerant pair to be suitable in a cascade refrigeration system, several criteria such as temperature range compatibility, performance, system compactness, etc., should be fulfilled.
Basic Properties of Refrigerants
Selection of suitable refrigerant mainly depends on its thermodynamic properties, physical properties and chemical properties. The refrigerant that satisfies all the desirable thermodynamic properties, physical properties and chemical properties can be called as ideal.
However, different refrigerants seem to satisfy different requirements and sometimes only partially. A refrigerant which is ideally suited in a particular application may be a complete failure in the other.
Hence, the refrigerant has to be selected for a certain application in such a way that the most of the basic properties should be favourable for that application. Required properties of the ideal refrigerant can be summarised as:
- The refrigerant should have low boiling point and low freezing point
- It must have low liquid specific heat, high vapour specific heat and high latent heat. This is because high specific heat decreases the specific refrigerating effect and high latent heat at low temperature increases the specific refrigerating effect
- The pressures required to be maintained in the evaporator and condenser should be low enough to reduce the material cost and must be positive to avoid leakage of air into the system
- It must have high critical pressure and temperature to avoid large power requirements
- It should have low specific volume to reduce the size of the compressor
- It must have high thermal conductivity to reduce the area of heat transfer in evaporator and condenser
- It should be non-flammable, non-explosive, non-toxic and non-corrosive
- It should not have any bad effects on the stored material or food, when any leak develops in the system
- It must have high miscibility with lubricating oil and it should not have reacting property with lubricating oil in the temperature range of the system
- It should give high COP in the working temperature range. This is necessary to reduce the running cost of the system
- It must be readily available and cheap.
Environmental Properties of Refrigerants
Due to severe global concern about ozone layer depletion and global warming, the environmental property plays a major role in refrigerant selection. Refrigeration and air conditioning system influences the environment in three aspects: (i) Ozone layer depletion, (ii) Global warming (natural disasters around the world have shown the negative effects of greenhouse gases including various refrigerants) and (iii) pollution due to production of consumed power (harmful to the health of people).
Today, we are again amidst a historical technological shift and this time the need to preserve our global environment is the main driving force. The Montreal Protocol (1987) was the first international agreement to set up a schedule for reduction and phase-out of the production and consumption of ozone depleting substances. The other important international scheme is the Kyoto Agreement, which 10 years later was established to reduce emissions of global warming gases.
Ozone layer depletion
The ozone layer absorbs most of the harmful ultraviolet-B radiation from the sun. It also completely screens out lethal UV-C radiation. Depletion of the ozone layer allows more harmful radiation to reach the earth, resulting in more melanoma and non-melanoma skin cancers, more eye cataracts, weakened immune systems, reduced plant yields, damage to ocean eco-systems and reduced fishing yields, adverse effects on animals and more damage to building materials and plastics.
The ozone hole and ozone depletion result from CFCs, HCFCs, Halons, methyl bromide and other ODS released to the atmosphere. The Ozone Depletion Potential (ODP) reflects the combination of percentage (by weight) of chlorine atoms and the lifetime of the compound in the atmosphere.
Global warming potential
Greenhouse gases have the potential to increase the earth’s average temperature by trapping some of the heat that the earth normally radiates back into space. CO2, methane, nitrous oxide are the three main greenhouse gases. Fluorinated compounds such as HFCs, PFCs and SF6 are also greenhouse gases. GWPs (Global Warming Potential) are used to compare the impact on the climate system of emission of different greenhouse gases with respect to carbon dioxide (normalized at 1).
As greenhouse gases differ in their atmospheric lifetimes, GWPs also have a time component. Time horizons of 20 years and 100 years are used in this document to enable the proper evaluation on the environment. The economic and environmental effects considered were refrigerant supply cost, direct Global Warming Impact (GWI), and Total Equivalent Warming Impact (TEWI).
Direct Global Warming Impact (GWI) is the direct effect of released refrigerant emission in equivalent tonnes of carbon dioxide. The GWI is an annual figure obtained by multiplying annual emissions by the reported Global Warming Potential (GWP) of the species. Total equivalent warming impact per annum (TEWI) is the sum of the GWI and the carbon dioxide emitted in the production of energy to run the system (TEWI = direct + indirect emission). Research on TEWI (Total Equivalent Warming Impact) has shown that for most applications the impact on global warming will be greater from energy consumption than from CO2 equivalent emission (release) of refrigerants.
According to the humanities point of view, the NYAY (Justice) is the use of such technology (here, refrigerant and it system), which is not only human friendly, but also environment or biosphere friendly. It has been shown that all the natural refrigerants (naturally occurring substances) have negligible negative effect on the environment or biosphere. Furthermore, there is no need of production of natural refrigerants, and hence no production related pollution or negative effect. Hence, natural refrigerants can be assumed as next generation refrigerant for cooling and heating applications.
Key Performance and Design Parameters
Coefficient of performance (COP)
COP is generally taken as a main performance parameter in refrigerant selection for any refrigeration and heat pump applications, and hence, obviously for cascade refrigeration system also. Main reason behind that, the higher COP yields lower running cost for certain cooling or heating load. COP is also considered as thermodynamic property of refrigerant as it is mainly dependent on basic refrigerant properties. For cascade refrigeration system, COP is calculated as the ratio of cooling capacity/load and the total/overall electrical energy input through all compressors. Hence, the maximum COP is an essential criterion of refrigerant pair selection for cascade refrigeration system.
Volumetric cooling capacity
The volume of suction vapour required per unit of refrigeration is an indication of the size of compressor. Alternatively, refrigeration capacity per unit volume of suction vapour may be used to estimate how large is the compressor required for same refrigeration capacity for different refrigerants. Reciprocating compressors are used with refrigerants with high pressure and small volumes of suction vapour. Centrifugal or turbo compressors are used with refrigerants with low pressures and large volumes of suction vapour. For a single stage vapour compression cycle, refrigeration capacity per unit volume of suction vapour can be used to estimate the size of compressor and a comparison can be made between various refrigerants.
However, for a cascade system, there are two compressors one in HT circuit and another in LT circuit. Size of a compressor is directly related to its initial manufacturing cost and upto some extend to the running cost (for a reciprocating compressor there will be higher frictional losses for a large sized compressor and more power is required to run it as compared to a small sized compressor). So compressor size should be as small as possible for unit refrigeration capacity. In view of increasing demand of cooling density (cooling capacity per unit volume), it is now an important criteria.
Heat exchanger size
For design and selection, minimisation of initial/installation cost is equally important as running cost. Installation cost is mainly dependent on size of major components (if we assume that similar materials are applicable for all working fluids), specifically, here, compressors, condensers, evaporators and cascade heat exchangers. Volumetric cooling capacity is related to compressor’s size. Combined volumetric refrigeration capacity has been introduced by the author and his group for cascade system, which is the ratio of cooling capacity and combined suction volume flow rate of all compressors.
For a certain cooling load, heat exchanger size is dependent of types, which is dependent on system application as well as heat transfer load. Based on operating condition, heat exchanger configuration and refrigerant properties, the sizes of condenser, evaporator and cascade heat exchanger can be evaluated for certain refrigerant pair. Hence, the sizes of heat exchanger have been also considered as important criteria for refrigerant pair selection.
Refrigerant Pair Selection Guidelines
The refrigerant pair selection procedure involves the following two major steps:
- Screening of refrigerants based on environmental and safety considerations. Sorting of refrigerant pairs based on its operating temperatures, which should be in-between its normal boiling point temperature and critical temperature.
- Optimisation of intermediate temperature leading to maximum cooling COP and final selection of refrigerant pair based on best system performance along with best volumetric cooling capacity.
Case Studies: Natural Refrigerant Pair
The natural refrigerants which have been considered here are: Ammonia, Carbon dioxide, Propane, Propylene, n-Butane, Isobutane, Ethane & Ethylene. Out of these 8 natural refrigerants, there is possibility of using each refrigerant as HT fluid or LT fluid, thus giving rise to 56 possible combinations of HT fluid and LT fluid.
However, few pairs have sorted in initial screening and final screening has been made based on maximum COP and maximum volumetric cooling capacity. All the selections have been made based on an optimum intermediate temperature for an individual pair. Two selection charts for given Evaporator Temperature (TE) and Condenser Temperature (TC) have been developed, one for the highest COP (Figure 2) and another for the highest volumetric capacity (Figure 3).
