Engineering - Royal Australian Navy

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Naval <strong>Engineering</strong> Bulletin • June 2001 Air Conditioning Load Determination Load determination is dependent on the typical factors experienced in buildings however the following considerations are peculiar to the design of air conditioning systems on ships: • Heat gain from piping, machinery and equipment. • Sun load must be considered on all exposed surfaces above the waterline. If a compartment has more than one exposed surface, the surface with the greatest sun load is used and the other exposed boundary is calculated at outside ambient temperature. • It must also be borne in mind that unlike buildings where occupants come and go and as a result lighting/equipment is turned off and on, which results in a diversity factor when determining peak total cooling loads. This may not be as appropriate in ships where the people cannot leave and lighting/equipment is typically left turned on. • Infiltration through weather doors ie. doors that open to external walkways or decks is considered negligible however they may require an assumed infiltration load for heating steering gear rooms and air conditioning the bridge. As there are minimal windows or those that exist are watertight, infiltration is also regarded as being minimal. • When calculating winter heating loads, heat transmission through boundaries of machinery spaces in either direction is not considered. • For merchant ships, the cooling coil leaving air temperature is assumed to be 90C Dry Bulb and the Wet Bulb temperature is consistent with 95% relative humidity. This may be changed in summer if humidity control is deemed necessary. • The overall heat transfer coefficients for the composite structures common to ship construction do not lend themselves to theoretical derivation they are most commonly obtained from full-scale panel tests. • Heat dissipation from people typically depends on their activity levels and the ambient dry bulb temperature however values that can be used from ASHRAE are quoted in Table 1 as follows; Table 1. Heat Gain from Occupants in Watts Activity at 270C Sensible Latent Total Eating 64 97 161 Moderate Activity 59 73 132 Light Activity 57 60 117 Workshops 73 149 222 Table 2. HMAS IPSWICH and WHYALLA Temperature Comparison 57
Naval <strong>Engineering</strong> Bulletin • June 2001 Painting systems can also play an important part in the heat transmission as per the following case study however this is generally not a significant percentage of the total air conditioning load for larger ships. DSTO undertook a study comparing HMAS Ipswich was chosen for comparison with HMAS Whyalla as both ships have similar paint systems differing only in the topcoat application. Whyalla has a low gloss NIR reflecting Haze Grey polyurethane on all surfaces above the waterline, including a non-skid application on the decks. Ipswich has the current high gloss Storm Grey topcoat on surfaces above the waterline except the decks that are coated with the RAN dark grey Pewter non-skid coating. As both ships were painted in late 1996, it provides an excellent opportunity to compare paints of similar condition. The following table demonstrated the significant temperature difference. Air Conditioning System Components It is desirable as with any air conditioning system to group together those compartments having similar air conditioning or ventilation requirements. As in a building each ship has fire zones through which it is not desirable to penetrate ducting. Also ships have watertight bulkheads which it is also not desirable to penetrate ducting. As mentioned previously there is a desire to save weight and volume in the selection of equipment so when combined with the previously stated requirement generally results in piping delivering chilled water and /or steam to various strategically placed fan/coil units. These then provide air conditioning via direct supply to the compartment or via ducting to several compartments using the passageways and stairwells as return air paths. The other option is to install reverse cycle refrigerative plant for air conditioning on smaller ships or dedicated smaller compartments provided there is a suitable place to put the condensers. In general however equipment used for ships should be considerably more rugged than equipment for land applications, as highlighted previously it has to withstand a more aggressive environment which is both more corrosive and subject to shock loads. The shipbuilder, system designers and the owners of course determine this degree of ruggedisation. Air Distribution Good air distribution can be difficult because of low ceiling heights and generally compact space arrangements. To overcome such difficulties mock-ups or computational fluid dynamics techniques can be employed to determine design criteria for the location of ceiling diffusers. Naturally there is a need to minimise drafts and noise. There is also the possibility of condensation when the temperature difference between the initial space temperature and the discharge air temperature is too great. Air distribution can also be undertaken by the use of an induction system however these systems have experienced operational difficulties and have not been as successful. Air is typically returned via sight proof louvres in a doorway to the passageway. Duct is either manufactured from galvanised steel sheet or aluminium or for larger ducting for ventilation of engine rooms etc it is steel sheet. Air Quality in Ships There is a significant body of experience relating to air quality problems encountered in the land based environment due to air conditioning however on board ships the problems can also be widely variable depending on the: • Type of vessel; • Age of vessel; • Area of operation - climate, ambient air quality etc; • Period and density of passenger occupation; • System design and maintenance. Generally pollutants are more limited on board ships as stated previously. Lloyd’s register undertook a survey on board ships, which attempted to identify air quality problems. Carbon dioxide concentrations which can be used as a measurement of ventilation efficiency indicated that whilst there were high levels these were generally only peaks and overall ventilation efficiency was satisfactory Volatile organic compound levels were found to be comparatively low on board ships as compared to homes and offices. Particulate measurements were found to be particularly high compared to homes and offices however this may have been due to higher airflows causing re-suspensions of particulate matter. 58
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Engineering - Royal Australian Navy

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