Ashrae Journal - December 2008 - (Page 58) regulates refrigerant flow to the evaporator. An undersized expansion valve will prevent sufficient refrigerant flowing into the evaporator causing reduction in design cooling capacity of the system and an oversized expansion valve will allow too much refrigerant flow into the evaporator causing liquid refrigerant to flow back to the compressor. If not remedied, both conditions will cause compressor damage. Evaporator. Shell-and-tube type DX-evaporators are used for chiller packages. In DX-evaporators, fresh water flows on the shell side and refrigerant flows through tubes. Internally serrated tubes are used to increase heat transfer rate and make evaporators compact. The optimum design of an evaporator is done considering velocity, pressure drop and heat transfer rate to achieve saving in evaporator manufacturing and operating cost. When preparing the general arrangement and layout drawings, it is important to provide enough space for maintenance and tube cleaning of the evaporator. Air-Cooling Units. The refrigerant flows in the tubes and air flows over the fins of air-cooling units. Different types of compact air-cooling units are available. In a refrigeration plant its location and mounting must be carefully selected to obtain the desired cooling effect in cold storages. The air-cooling units must be installed inside rooms so that when doors are open, ACUs will not throw conditioned air outside the room and the ACU fan can’t suck in outside air easily. Provision of defrost heaters must be made for low temperature application. Auxiliary Systems. Auxiliary systems like a chilled water/ cooling water system is inherently required with vapor compressor refrigeration systems. An optimization approach in designing these systems helps in integrating these systems with the refrigeration system and the ship’s existing water system to reduce overall cost. Design the condenser for higher cooling water temperature difference, which will mean less flow rate and less pumping power for the same capacity. However, this makes the condensing temperature rise resulting in higher compressor power. The temperature rise of 4°C to 6°C (7°F to 11°F) is normally considered. For chilled water, as well as cooling water circuit, minimizing bends and elbows reduces the pressure drop in water piping. Selecting large diameter and shorter pipes reduces the pressure drop. Selecting large pipe size is costly and further water line valves also becomes costlier with increasing pipe size. Selection of appropriate pumps is also required to minimize operating cost, reducing cavitations, vibrations and noise. Piping and Fittings. The refrigerant, chilled water and cooling (sea) water piping must be selected by considering material compatibility. The sizes and ratings must be selected to ensure safe operation of the plant for the most expected severe design pressures and temperatures. Avoid pipe threads and reduce number of joints. All debris and weld slag must be properly cleaned and care should be taken so that it will not enter the piping. Ensure that brazed and welded joints are leak-proof by pressure testing. 58 ASHRAE Journal Testing The naval refrigeration plants and chiller packages undergo stringent type test, production test, endurance test, and tilt test, shock test, vibration and noise test, workshop test, and onboard trial. Usually, a navy publishes the specifications, procedures, and requirements for all such tests (statement of technical requirement) along with tender document. The supplier has to prove the plant performance against these requirements. Remember, that the refrigeration and air-conditioning plants are always oversized, proving the performance does not merely mean obtaining the guaranteed refrigeration capacity. However, it must be ensured that the performance is achieved with less than or equal to the guaranteed power consumption while meeting all other peformance parameters such as vibration, noise, etc. Testing ensures reliability of equipment and systems. Summary Most of the design parameters and constraints for designing naval refrigeration plants and chiller packages are presented in this article. An attempt is made to explain the designing procedure used and general tips are provided to unfold the secret of better design of naval refrigeration plants and chiller packages. Better equipment, standards, procedures, softwares, and design guidelines are becoming available within the industry, however, the future of naval refrigeration will remain subjective depending upon the method of application of these resources, judgment of designers, and improvement in technology. References 1. International Organization for Standardization. 2002. ISO 7547:2002, Ships and Marine Technology—Air-Conditioning and Ventilation of Accommodation Spaces—Design Conditions and Basis of Calculations. 2. Ministry of Defense, U.K. BR 3021, Parts 1 and 2. 1995. Shock Manual. 3. U.S. Department of Defense. MIL-STD-740-2. 1986. Structureborne Vibratory Acceleration Measurements and Acceptance Criteria of Shipboard Equipment. 4. U.S. Department of Defense. MIL-STD-167-1. 1974. Mechanical Vibrations of Shipboard Equipment. 5. U.S. Department of Defense. MIL-STD-1474D. 1997. Noise Limits. 6. Ministry of Defense, India. EED-Q-071(R3). 2007. “Specification of motors and starters for naval ships.” 7. Ministry of Defense, India. JSS 55555. 2000. Environmental Test Methods for Electronic and Electrical Equipment. 8. U.S. Department of Defense. MIL-STD-461D. 1993. Requirement for the Control of Electromagnetic Interference Emmissions and Susceptibility. Bibliography Gokhale A. 2008. “Chillers for warships.” ISHRAE Journal January–March:73 – 82. Acknowledgments The author wishes to thank Kirloskar Pneumatic Company Limited, Pune, India, for providing an opportunity to learn and design naval refrigeration and air-conditioning systems. ashrae.org December 2008 http://www.ashrae.org
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