The cooling load estimate considers the following factors (discussed in Chapter 18 of the 2017 ASHRAE Handbook—Fundamentals):

The heating load estimate should include the following:

In addition, the construction and transient nature of ships present some complications, as addressed in the following:

SNAME. The Society of Naval Architects and Marine Engineers (SNAME 2015) Technical and Research Bulletin 4-16 can be used as a guide for shipboard load calculations.

ISO. The International Organization for Standardization’s (ISO) Standard 7547 discusses design conditions and calculations for marine HVAC systems.

Outdoor Ambient Temperature and Humidity. The service and type of vessel determine the proper outdoor design temperature, which should be based on temperatures prevalent in a ship’s area of operation. Use Chapter 14 of the 2017 ASHRAE Handbook—Fundamentals to select ambient conditions, with special attention paid to high-wet-bulb data; a ship’s load is often driven by the latent load associated with the outdoor air. It is also common for different locations to be used for cooling and heating criteria. In general, for cooling, outdoor design conditions are 95°F db and 78°F wb; for semitropical runs, 95°F db and 80°F wb; and for tropical runs, 95°F db and 82°F wb. For heating, 0°F is usually the design temperature, unless the vessel will always operate in warmer climates. Design temperatures for seawater are 90°F in summer and 28°F in winter.

Solar Gain. Ships require special consideration for solar gain because (1) they do not constantly face in one direction and (2) the reflective properties of water increase solar load on outer boundaries not directly exposed to sunlight. For compartments with only one exterior boundary, the temperature difference (outdoor dry-bulb temperature – indoor dry-bulb temperature) across horizontal surfaces should be increased by 50°F and vertical surfaces by 30°F. For compartments with more than one exterior boundary, the temperature difference should be increased by 35°F for horizontal surfaces and 20°F for vertical surfaces. For glass surfaces, the solar cooling load (SCL) is taken to be 160 Btu/h · ft² for spaces with one exterior boundary and 120 Btu/h · ft² for spaces with more than one exterior boundary. A more modern approach is to use appropriate building energy simulation software (a full list of which is maintained by the U.S. Department of Energy) to model the ship’s accommodation spaces. These programs generally allow building exposure to be changed globally, making it easy to examine the change in ship loads as the route direction changes.

Infiltration. Infiltration through weather doors is generally disregarded. However, specifications for merchant ships occasionally require an assumed infiltration load for heating steering gear rooms and the pilothouse.

Transmission Between Spaces. For heating loads, heat transmission through boundaries of machinery spaces in either direction is not of consequence. Allowances are not made for heat gain from warmer adjacent spaces. For cooling loads, the cooling effect of adjacent spaces is not considered unless temperatures are maintained with refrigeration or air-conditioning equipment.

Ventilation Requirements. Ventilation is a very important consideration, because it is frequently the main contributor to overall energy usage of the system. Rules and guidance are provided by conflicting standards, including ISO Standard 7547, SNAME Technical and Research Bulletin 4-16, and ASHRAE Standard 62.1-2016. Ultimately, that stated in the ship’s specification and what is acceptable to the authority having jurisdiction (AHJ) govern ventilation requirements. However, given the opportunity to reduce energy (fuel) consumption and the need to ensure passenger health and safety, it behooves the system designer to apply modern tools, such as building energy simulation and demand-controlled ventilation to optimize the quantity of fresh air introduced under all conditions. There is a unique opportunity for fresh-air optimization aboard ships because the number of passengers is fixed and each person can only be in one location at a time. Significant overventilation that can occur by catering to the maximum fresh-air requirement in each space simultaneously.

Heat Transmission Coefficients. The overall heat transmission coefficients U for the composite structures common to shipboard construction do not lend themselves to theoretical derivation; they are usually obtained from full-scale panel tests. SNAME Bulletin 4-7 gives a method to determine these coefficients when tested data are unavailable. ISO Standard 7547 also gives some guidance in this area, as well as default values if better information is not available.

Indoor Air Temperature and Humidity. Thermal environmental conditions for human occupancy are given in ASHRAE Standard 55-2017.

People. Ships normally carry a fixed number of people. The engineer must select the location where the ship’s fixed complement of people creates the greatest heat load, and then not apply the people load elsewhere. Note that occupants are only counted once when determining the chiller or condensing-unit load; however, air coils in each zone must be capable of removing the heat load associated with the maximum number of people in the zone.

Ventilation in the zone can also be reduced when occupants are not present. For the ventilation load, occupants are counted once, in the location where they create the greatest ventilation requirement. The practical way to apply this concept is by measuring CO₂ levels in a space and adjusting outdoor air accordingly. Although using this principle can reduce required chiller or condensing-unit capacity on all ships, it is most significant for passenger ships.

In general, equipment used for ships is much more rugged than that used on land. Sections 6 through 10 of ASHRAE Standard 26 list HVAC equipment requirements for marine applications. When selecting marine duty air-conditioning equipment, consider the following:

The following general items should be considered when selecting specific air conditioning components:

Fans. Fans must be selected for stable performance over their full range of operation and should have adequate isolation to prevent transmitting vibration to the deck. Because fan rooms are often adjacent to or near living quarters, effective sound treatment is essential.

Cooling Coils. If more than 30% outdoor air is brought across a cooling coil, consider using copper tube, copper fin, epoxy-coated coils, or other special treatment. To account for the ship’s movement, drain pans should have two drain connections, and should ideally be dual sloping, with extra depth. Because of size constraints, care must be taken to prevent moisture carryover. Face velocity limits (in fpm) for different coil materials and different fin spacing are as follows:

Off-coil temperatures are another concern. Ships typically have low ceiling heights and cannot tolerate low air-introduction temperatures. Typically 55°F db and 54°F wb are used as limiting off-coil temperatures.

Electric Heaters. U.S. Coast Guard (USCG) approved sheathed-element heaters are typically required. The only exception is when the electric heaters, approved by a regulatory body such as UL, are incorporated in a packaged unit.

Air Diffusers. Care must be taken with selection of air diffusers because of the low ceilings typical of shipboard applications.

Air-Conditioning Compressors. Compressors of all types are used for marine applications. Care must be taken when using a centrifugal compressor because low-load, high-condensing temperature is a common off-load condition.

When high discharge temperatures are a concern, seawater-cooled heads are not normally an option; other methods such as fan cooling or liquid injection must be considered for maintaining acceptable discharge temperatures.

All types of systems may be considered for each marine application. The systems are the same as in land applications; the difference is the relative weighting of their advantages and disadvantages for marine use. This section does not review all the systems used aboard ships, but rather some of the more common ones.

Direct refrigerant cooling systems are often used for small, single-zone applications. Aboard ships, places like control rooms and pilot houses lend themselves to a direct refrigerant system. For larger spaces, air distribution is of more concern; direct refrigerant cooling is thus less likely to be the optimum solution.

Two-pipe and four-pipe fan coil systems are often used for large systems. The water piping used in these systems takes up only a fraction of the space used by an all-air ducted system. Fan noise in the space being cooled is the disadvantage. In addition, limited humidity control and fresh-air requirements often need to be addressed separately.

Many types of all-air systems are used aboard ships. Space, cost, noise, and complexity are among the leading parameters when comparing different all-air systems. Using high-velocity air distribution for an all-air system offers many advantages; unitary (factory-assembled) central air-handling equipment and prefabricated piping, clamps, and fittings facilitate installation for both new construction and conversions. Substantial space-saving is possible compared to conventional low-velocity sheet metal ducts. Maintenance is also reduced. Noise is the one major drawback of a high-velocity system, which often leads to selection of a low-velocity system.

Terminal reheat air conditioning (described in Chapter 4 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment) is commonly used because of its simplicity and good zone control characteristics. However, as systems become larger, this system’s energy inefficiency becomes a significant drawback. For large passenger ships, where energy efficiency is absolutely essential, the designer should consider more modern approaches. One method is delivering tempered ventilation air from a central-station air handler to the passenger staterooms, each of which is outfitted with a dedicated cabin fan-coil unit, including a chilled-water-cooling and hot-water-heating coil. The individual cabin is then either heated or cooled from a neutral temperature, using central loops, depending on specific needs. The efficiency of this approach is further improved by having reclaimed heat sources (e.g., from the central chillers or the main engine cooling system) to maintain the hot-water loop temperature.

Dual-duct systems (also described in Chapter 4 of the 2020 ASHRAE Handbook—HVAC Systems and Equipment) have the following advantages:

The major drawback is the inability to finely control temperature and humidity. This disadvantage is enough to preclude the use of these systems in many passenger vessel applications.

Aboard ships, constant-volume systems are most common. Their advantages include simplicity (for maintenance, operation, and repair) and low cost. However, for large passenger vessels, the energy efficiency and the tight control of zone temperature make variable-volume/temperature systems very attractive.

Good air distribution in staterooms and public spaces is difficult to achieve because of low ceiling heights and compact space arrangements. Design should consider room dimensions, ceiling height, volume of air handled, air temperature difference between supply and room air, location of berths, and allowable noise. For major installations, mock-up tests are often used to establish exacting performance criteria.

Air usually returns from individual small spaces either by a sight-tight louver mounted in the door or by an undercut in the door leading to the passageway. An undercut door can only be used with air quantities of 75 cfm or less. Louvers are usually sized for face velocity of 400 fpm based on free area.

Ductwork on merchant ships is generally constructed of steel. Ducts, other than those requiring heavier construction because of susceptibility to damage or corrosion, are usually made with riveted seams sealed with hot solder or fire-resistant duct sealer, welded seams, or hooked seams and laps. They are made of hot-dipped, galvanized, copper-bearing sheet steel, suitably stiffened externally. The minimum thickness of material is determined by the diameter of round ducts or by the largest dimension of rectangular ducts, as listed in Table 1.

The increased use of high-velocity, high-pressure systems has resulted in greater use of prefabricated round pipe and fittings, including spiral-formed sheet metal ducts. It is important that field-fabricated ducts and fittings be airtight. Using factory-fabricated fittings, clamps, and joints effectively minimizes air leakage for these high-pressure ducts.

In addition to the space advantage, small ductwork saves weight, another important consideration for this application.

The conditioning load, even on a single voyage, varies over a wide range within short periods. Not only must the refrigeration plant meet these load variations, but the controls must readily adjust the system to sudden climatic changes. Accordingly, it is general practice to equip the plant with automatic controls. Increasingly, fully communicating network controls are being applied to optimize operation of the entire system under transient conditions. Moreover, such controls now can collect large amounts of operational data and offer proactive diagnostics based on these data, helping the ship operator maximize efficiency and reliability throughout the life cycle.

Merchant vessels that operate under the U.S. flag come under the jurisdiction of the U.S. Coast Guard. Accordingly, the installation and components must conform to the Marine Engineering Rules and Marine Standards of the Coast Guard covered under the Guide to Structural Fire Protection (USCG 2010).

Certified pressure vessels and electric components approved by independent agencies (e.g., ASME, UL) must be used. Wherever possible, equipment used should comply with ABS rules and regulations. This is especially important when vessels are equipped for carrying cargo refrigeration, because air-conditioning compressors may serve as standby units in the event of a cargo compressor failure. This compliance eliminates the need for a separate, spare cargo compressor. The International Convention for the Safety of Life at Sea (SOLAS) (IMO 2014) governs the use of fire-dampers and duct wall thickness when passageways or fire boundaries are crossed.

Outdoor Ambient Temperature. Design conditions for naval vessels have been established as a compromise, considering the large cooling plants required for internal heat loads generated by machinery, weapons, electronics, and personnel. Temperatures of 90°F db and 81°F wb are used for worldwide applications, with 85°F seawater temperatures. Heating-season temperatures are 10°F for outdoor air and 28°F for seawater.

Indoor Temperature. Naval ships are generally designed for space temperatures of 80°F db with a maximum of 55% rh for most areas requiring air conditioning. USN (1969) gives design conditions established for specific areas, and USMA (1965) lists temperatures for ventilated spaces.

Ventilation Requirements. As for merchant ships, there is conflicting guidance regarding ventilation requirements; see ISO Standard 7547, SNAME Technical and Research Bulletin 4-16, and ASHRAE Standard 62.1-2016.Ventilation must meet the requirements of the ship’s specification and the U.S. government, but as with merchant ships, the system designer should apply modern tools, such as building energy simulation and demand controlled ventilation, to optimize the quantity of fresh air introduced under all conditions, ensuring safe operation and technical justification.

Air-Conditioned Spaces. Naval ship design requires that air-conditioning systems serving living and berthing areas on surface ships replenish air in accordance with damage control classifications, as specified in USN (1969):

The cooling load estimate consists of coefficients from USN’s Design Data Sheet DDS511-2, General Specifications for Building Naval Ships, or Document 0938-018-0010 (USN 1969) and has allowances for the following:

Loads should be derived from requirements indicated in USN (1969). The heating load estimate should include the following:

Some electronic spaces listed in USN (1969) require adding 15% to the calculated cooling load for future growth and using one-third of the cooling-season equipment heat dissipation (less the 15% added for growth) as heat gain in the heating season.

Heat Transmission Coefficients. The overall heat transmission coefficient U between the conditioned space and the adjacent boundary should be estimated from USN’s Design Data Sheet DDS511-2. Where new materials or constructions are used, new coefficients may be used from SNAME (2015) or calculated using methods found in DDS511-2 and SNAME.

Heat Gain from People. USN (1969) gives heat gain values for people in various activities and room conditions.

Heat Gain from Sources in the Space. USN (1969) gives heat gain from lights and motors driving ventilation equipment. Heat gain and use factors for other motors and electrical and electronic equipment may be obtained from the manufacturer or from Chapter 18 of the 2017 ASHRAE Handbook Fundamentals.

The equipment described for merchant ships also applies to U.S. naval vessels, except as follows:

Fans. A family of standard fans is used by the navy, including vaneaxial, tubeaxial, and centrifugal fans. Selection curves used for system design are found on USN’s NAVSEA Standard Drawings 810-921984, 810-925368, and 803-5001058. Manufacturers are required to furnish fans that are dimensionally identical to the standard plan and within 5% of the delivery. No belt-driven fans are included.

Cooling Coils. The U.S. Navy uses eight standard sizes of direct-expansion and chilled-water cooling coils. All coils have eight rows in the direction of airflow, with a face area range of 0.6 to 10.0 ft².

Coils are selected for a face velocity of 500 fpm maximum; however, sizes 54 DW to 58 DW may have face velocity up to 620 fpm if the bottom of the duct on the discharge is sloped up at 15° for a distance equal to the height of the coil. Construction and materials are specified in MIL-PRF-2939G.

Chilled-water coils are most common and are selected based on 45°F inlet water with approximately a 6.7°F rise in water temperature through the coil. This is equivalent to 3.6 gpm per ton of cooling.

Heating Coils. The standard naval steam and electric duct heaters have specifications as follows:

Steam Duct Heaters

Electric Duct Heaters

Filters. Characteristics of the seven standard filter sizes used by the U.S. Navy are as follows:

Air Diffusers. Although the U.S. Navy also uses standard diffusers for air conditioning, they are generally a commercial type similar to those used for merchant ships.

Air-Conditioning Compressors. In the past, the U.S. Navy primarily used reciprocating compressors up to approximately 150 tons; for larger capacities, open, direct-drive centrifugal compressors are used. On new designs, the U.S. Navy primarily uses rotary compressors (e.g., screw and centrifugal), frequently semihermetic. R-134a is the U.S. Navy’s primary refrigerant. Seawater is used for condenser cooling at 5 gpm per ton for reciprocal compressors and 4 gpm per ton for centrifugal compressors, but in all cases, the maximum seawater velocity of 6 fps must be deferred to in order to prevent tube erosion.

On naval ships, zone reheat is used for most applications. Some ships with sufficient electric power use low-velocity terminal reheat systems with electric heaters in the space. Some newer ships use a fan-coil unit with fan, chilled-water cooling coil, and electric heating coil in spaces with low to medium sensible heat per unit area of space requirements. The unit is supplemented by conventional systems serving spaces with high sensible or latent loads.

Methods used on naval ships are similar to those discussed in the section on Merchant Ships. The minimum thickness of materials for ducts is listed in Table 2.

The navy’s principal air-conditioning control uses a two-position dual thermostat that controls a cooling coil and an electric or steam reheater. This thermostat can be set for summer operation and does not require resetting for winter operation.

Steam preheaters use a regulating valve with (1) a weather bulb controlling approximately 25% of the valve’s capacity to prevent freeze-up, and (2) a line bulb in the duct downstream of the heater to control the temperature between 42 and 50°F.

Note: All dimensions in inches.

Other controls are used to suit special needs. Pneumatic/electric controls can be used when close tolerances in temperature and humidity control are required, as in operating rooms. Thyristor controls are sometimes used on electric reheaters in ventilation systems.

Modern ship designs use fully communicating networked controls that optimize system operation and provide useful data and feedback to the operator. The Navy is increasingly implementing energy-saving measures aboard all of its ships.