Ashrae Journal - December 2008 - 51

side. Hatches, openings, passages, and staircases interconnect the compartments. The refrigeration plants and chiller packages are normally installed in any of these small compartments located in forward, aft (rear), and the mid portion of a ship, depending upon function. These compartments contain additional equipment, pipelines, and electrical cables, restricting the available space. A feasibility study considering the following points is essential for retrofits and is useful when designing new plants. The plant and equipment must be compact enough to be accommodated in the available space. The shipping route of individual equipment and/or a complete plant must be considered before designing the individual equipment or complete plant. The plant’s orientation within the compartment is decided after carefully considering its operational and maintenance requirements, and the best use of existing pumps and piping and electrical cabling. The mounting arrangement is decided by considering the foundation, bulkhead, deck head supports, and stiffness. Once the sizes, weights, and operational requirements of individual equipment are known and, after considering all the previous points, the positioning and assembling of the individual equipment is done to form a compact skid to fit in the available space. Equipment Weight in different directions, periodically recording operational and safety cutout parameters. Environmental Conditions Instruments and controls must be selected considering the range of temperature and pressure encountered during ship operation. For example, if seawater is used as the cooling medium in a condenser, the range of thermostatic expansion valves must be properly selected considering the variation in seawater temperatures. As in the case of extremely low seawater temperature, the pressure drop across an expansion valve may not be sufficient for desired flow rate of refrigerant. Seawater-cooled condenser design must consider these conditions. The seawater pumps, piping, valves, fittings, and flow meters must be properly selected considering these changing environmental conditions. Instruments such as some flow meters need recalibration with changing environmental conditions. During sailing, this is difficult, care must be taken when choosing suitable and proven instruments. Shock The equipment designed for naval use is expected to be lighter and stronger. The size and material of construction decides the weight of the equipment. Reduction in weight is possible by reducing the size and using lighter construction materials, keeping its operational and functional requirements intact. Equipment such as heat exchangers and pressure vessels carry operating fluids. The weights for the equipment are specified as empty and operational in their datasheets. Standard engineering practices and international standards from the American Society of Mechanical Engineers (ASME), Tubular Exchanger Manufacturers Association (TEMA) and ASHRAE assist in optimizing the size and weight of such equipment. Seaway Conditions Warships and submarines are mainly exposed to two types of shocks: 1) underwater explosion (underwater shock) and 2) the reactive force caused by firing a missile from the ship. The underwater shock is critical. Severity of shock depends upon magnitude of the explosion, location of the equipment and its supports, and duration and propagation of the shockwave. Each navy specifies different shock grade environments for various types of equipment on a case-by-case basis. The equipment and machinery supplied for naval use must be able to withstand the specific shock and perform satisfactorily after a shock. Effects of Shock The refrigeration plants and chiller packages are designed to operate satisfactorily under the seaway conditions of rolling, pitching, heave, yaw, tilt, list, and trim. Seaway conditions are an important consideration when locating and positioning plants within the compartments. Advanced computer analysis can assist in simulating and predicting the equipment performance under equivalent seaway conditions. Before designing, it is important to understand the flow behavior of fluids in the equipment under seaway conditions. The major causes of failure/unsatisfactory performance of refrigeration plants and chiller packages under seaway conditions are insufficient oil flow to compressor, improper separation of oil in an oil separator, malfunctioning of level switches, and inefficient heat exchange in heat exchangers. These aspects are considered by performing an actual tilt test December 2008 Shock has the potential for damaging the plant or the plant’s equipment. • Brittle or fragile items can fracture and ductile items can bend due to shock. Some items may not be damaged by a single shock but will experience fatigue failure with many repeated low-level shocks. • Shock may result in only minor damage, which may not be critical. However, cumulative minor damage from several shocks will eventually result in the equipment/ plant being unusable. • Shock may not produce immediate apparent damage but might cause the service life of a part such as a compressor to be shortened, and reliability is reduced. • Shock may cause parts to fall out of adjustment (e.g., couplings between compressors, motor, some precision instruments, etc.). Designing for Shock Design using shock calculations. A navy will specify the shock environment for a particular class of ship and submarine and provide the related shock grade curve. The amplitude and duration of shock ASHRAE Journal 51

Ashrae Journal - December 2008

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