Individual components of the systems remain same but the system adapts to the application’s requirements. In order to understand systems better, it is necessary to understand their classification and how individual components affect system performance.
Refrigeration systems are classified based on their Evaporator and Condensers as:
Comparison of performance of condensers
Condenser controls pressure and therefore capacity of the compressors. Final heat rejection to atmosphere is in the condensers and as can be seen above, it could be in the form of air or cooling water.
In case of very low temperatures, cascade systems are used where the intermediate condenser could be a refrigerant liquid fill tank. When the refrigerant gas coming out of Low Stage compressor is mixed into the refrigerant liquid, the liquid evaporates and cools the gaseous refrigerant. This liquid is then taken to the suction of high stage compressor and finally the heat is rejected to atmosphere through its condenser, which would be of one of the three types mentioned in next page.
Application of Dx type of systems in Industry
Dx systems are used in various applications as mentioned herein –
-Chemical processes where refrigerant is directly expanded in the heat exchangers or reactors (Fertilizer plants).
-Tank farms for storage of LPG or Propane where the fl uid cooled is the fluid compressed
-Ice cream tunnels or milk chillers
-Cold rooms and blast coolers
-Cryogenic applications
-Industrial HVAC systems.
Advantages of Dx over systems using heat transfer mediums like chilled water / brine are as follows
-Lesser heat transfer area is required-
-Evaporating liquid has better heat transfer coefficients
-For the same energy, temperature of refrigerant is lower than that of chilled water/ brine
-Rate of cooling is very fast
-Thermal storage in the refrigerant is higher compared to same volume of water
-No chilled water pumping.
Disadvantages of Dx systems are as follows
- Pressure drop in the liquid refrigerant lines is large if the distance of user equipment is high. In order to overcome it, a separate pump is required, resulting in energy loss
- Over the liquid and gaseous pipe lines, refrigerant picks up heat resulting in superheating again causing energy loss
- Possibility of refrigerant leakages cause loss of refrigerant
- Variation in user load pattern forces the equipment to part load without ON/OFF control.
Application of Medium type of systems in Industry
These are most commonly used systems in the industry as they off er tremendous flexibility in catering to multiple requirements at different locations. Since there is a medium involved in it, handling of the medium (chilled water /brine) is equally important as that of refrigeration system. Power consumption of the pumps required for circulation of the medium are additional and particularly in case of low load on the system, pumping power becomes significant.
Chilled water systems are further sub-divided into
Closed loop systems
Closed loop system is when the chilled water is circulated through the system and chiller by a single (circulation) pump. In this kind of system, pumping is most optimized and the minimum (and maximum) fl ow in the system is equal to minimum (and maximum) fl ow handled by the refrigeration system.
Closed loop system is effective in following cases:
- Similar temperature need (like HVAC system)
- Flow variations are not large
- Thermal shock to the system is not large
- Demand is continuous and at an average load.
Hot Well – Cold Well system
These systems are used in cases where the closed loop systems are not effective or technically challenged. They are preferred by users as operating variations do not affect the basic refrigeration system. However, it is an energy inefficient system and power consumption of pumping is generally higher than closed loop system.
Conclusion
Refrigeration systems are used in various applications in Industrial environment. Though not entirely different, applications decide arrangement of the components andtheir inter connectivity.
Performance of the system is at its best when the application matches equipment design and therefore it is equally important to do proper application engineering as it is to select right equipment.
