Air conditioning: Practicalities by Andrew Bray

PRACTICALITIES BY ANDREW BRAY

Staying cool below decks depends on good ventilation and other preventative measures, but when temperatures really rise it is often not enough.

Once you’ve sorted out the basics that can have a dramatic effect, such as using light external colours, awnings, insulation, reflective window films, then its time to look at what technology can offer. Even after deploying fans to create local breezes and using them to expel hot air from the engine space and the galley, there are still occasions when most of us just wish we could close the hatches and turn on air conditioning. It needn’t be just a dream and there are various options.

Heat pumps
Virtually all boating AirCon systems are based on the same mechanical “heat pumping” principle used in refrigeration. An understanding of this process is helpful when considering the options. Practical heat pumps have many refinements, but essentially there are just four components - evaporator, compressor, expansion orifice and condenser. These are connected by copper pipework to form a closed circuit, circulating a refrigerant fluid. See fig.1. Both the evaporator and the condenser are heat exchangers containing coils of copper tubing. Heat flows through the copper between the refrigerant and the air or water on the outside of the coils. Heat transfer to air is assisted by fins on the outside of the tubes.

Inside the evaporator, which is connected via a pipe to the suction inlet of the compressor, pressure is kept so low that liquid refrigerant boils into vapour at very low temperatures. The latent heat needed to vaporise the refrigerant is obtained by lowering its own temperature, helped by the surrounding cabinet; similar to the way alcohol cools your hand as it evaporates. After being sucked into the compressor, the initially cold, low pressure vapour is forced at relatively high pressures into the condenser. The vapour gets very hot during this compression (just as the air in a bicycle tire pump does), but at such pressure the refrigerant readily condenses back to a warm liquid as the heat is taken out of it by air or water cooling the condenser. The condensed liquid refrigerant then squirts back into the evaporator via a fine orifice or tube to go through the cycle again. Latent heat acquired at low temperature in the evaporator has been “pumped” into the air or water cooling the condenser, at higher temperature.

Many AirCon systems are termed “Reverse Cycle” because they contain valves that can reverse the roles of the evaporator and condenser, warming instead of cooling the boat. In either direction, the process is surprisingly efficient, "pumping" several times more heat energy in either direction than is input as work-energy to the compressor.

Practical systems
Practical heat pumps incorporate in-line driers and filters (to prevent ice or dirt blocking the expansion orifice) and also circulate compressor lubricant. Rather than a fixed orifice, larger systems may have an expansion valve that automatically regulates liquid refrigerant flow into the evaporator depending on its temperature. Many materials may be used as refrigerants, but in air conditioning the ozone depleting R22 is to be phased out, so many new systems use more environmentally friendly refrigerants such as R410 or R407. Controls automatically shut the system down if the compressor discharge pressure gets too high (or too low in some cases), or if an electric drive motor overheats. Most marine air conditioning systems are based on hermetically sealed 230V AC powered motor/compressor units, similar to those in domestic AirCon and refrigeration systems, and are similarly low maintenance. Very high capacity compressors can be belt driven by an engine, but such systems are more complex and tend to require considerable maintenance.

Controls
AirCon systems incorporate one or more thermostats (sometimes with wireless or infrared remote controls), which regulate cabin temperature by stopping and starting the compressor. Or in some installations the flow of refrigerant, chilled air or water. There’s also a main on-off switch, and heat-cool selection for reverse cycle systems. Cabin air is recirculated via inlets and outlets called registers, which may incorporate louvred or sliding mechanisms that allow the air flow to be regulated or directed. Some registers have their own booster fan with variable speed settings and most incorporate dust filters.

Humidity reduction
As well as lowering air temperature, AirCon tends to lower absolute humidity, as moisture condenses out of the air onto the evaporator fins and drains into a sump. If the relative humidity also falls this assists natural cooling through our skin but condensing moisture is otherwise a waste of energy. Freezing of moisture is a further waste and interferes with heat transfer so a thermostat cut-out is usually fitted to air cooled evaporators to avoid icing.

Others systems don’t need this because they don’t use the evaporator to cool air directly which leads us to consider the range of options in boating AirCon systems.

Equipment options
Air conditioning units can be self-contained devices incorporating all their mechanical components within one package or have them separated in various configurations. Most types are available in kit forms in various capacities and physical configurations to suit different installation situations. Kits are generally very compact and many are pre-charged with refrigerant and have self-sealing connections in refrigerant lines if necessary. More complex systems may require installation by a refrigeration mechanic. Installing, equipment, ducts and pipework is best done during vessel construction because retro-fitting can be challenging, especially on smaller boats.

Air-cooled systems
Self-contained, free standing air-cooled AirCon packages are familiar to everyone, as the ubiquitous window or wall-mounted domestic unit. Their relatively bulky condensers are cooled by outside air and a large fan. In windless tropical marinas, boat owners sometimes mount domestic units over or beside a deck hatch, with weather protection and ducting into the cabin. Small air-cooled units are available for mounting over a caravan or RV hatch, typically featuring a stylish streamlined nacelle. Designed for external overhead mounting, and powered via an extension cord, these are “off-the-shelf” solutions. If there’s sufficient shore power and AirCon isn’t required away from the marina, this works very well. There are marine versions, including some with carry handles. The need to stow the gear before going to sea can even be got around by installing within a cockpit locker and ducting the external air via sealable ports. However, most boating AirCons don’t use air-cooled condensers.

Water-cooled systems
Water-cooled condensers are physically very small and in many cases are simply one copper pipe within another, the pair coiled compactly around the hermetically sealed compressor. The electric pump providing the flow of cooling water is generally quiet and needs less power than an equivalent fan driven air-cooled system. High seawater temperature will degrade the unit’s efficiency, requiring more power, but water-cooled systems remain effective in the tropics. Reverse cycle heating systems may lose capacity at seawater temperatures below about 5oC, making them unsuitable for really cold water cruising.
There are three broad categories of marine, water-cooled equipment – package systems, split systems and multi-cabin systems.

Package systems
Package water-cooled systems require just an electrical power source for the compressor, cooling water inlet and discharge connections, and a condensation drain to a bilge or a shower sump - not in the engine space, since fumes may be drawn up the drain into the cabin. Given a suitable locker, package systems are readily retro-fitted, and are typically mounted low down within the cabin, taking in air from near sole level and discharging cooler air via a duct to an outlet near the deckhead. This is the commonest form of AirCon on small to medium-sized yachts. One unit can be installed to cool either of two adjacent spaces, such as the saloon and a forepeak sleeping cabin, or alternative outlets within the one cabin. Air flows are switched by damper valves, sliding closures, or louvred outlet fittings, but unless the electrics are customised, the thermostat will only be in one place. In-line booster fans may help, but cooling widely separated cabins generally requires two-package systems.

Split systems
The next step up is to place the evaporator and fan where the cooling is needed, but the rest of the system in the engine room or a lazarette. Copper refrigerant pipes link the two components, and the evaporator still needs a drain, but split systems allow greater flexibility in the siting of registers (outlets).

Multi-cabin systems
Split systems with separate evaporators in different cabins, each with its own thermostat can be served by the one large compressor/condenser set. Kits are available for this too, but installation requires a refrigeration mechanic. Other approaches are to have just one large evaporator, cooling air or water, and then distribute the cooling effect by circulating the chilled air or water around the boat.

Ducted cold air
Ducted cold-air systems suit larger commercial vessels with continuously running engines, but the bulky ductwork and associated booster fan noises count against them in smaller craft. An exception may be larger centre cockpit yachts where the heat pump might be within the engine space beneath the cockpit with relatively short ducts to adjoining wheelhouse, aft cabin, and saloon. Apart from the undesirability of having evaporators within hot engine spaces, ducted air systems can also be difficult to control, since opening or closing a register in one cabin effects the flow of air to the others.

Chilled water
In these systems, the evaporator chills water containing antifreeze, which is distributed by a circulating pump in parallel to heat exchangers in different cabins. The heat pump (with insulated evaporator) can be sited in the engine space and kits installed without needing a refrigeration mechanic. The low pressure plastic distribution pipework is small and flexible enough to be accommodated behind linings or beneath the sole and can be sealed where it passes through watertight bulkheads. The cooling effect can be quietly delivered right to where it is needed and is transferred to the cabin’s air in a unit often called an air-handler, which incorporates a fan and water/air heat exchanger. Each air-handler functions much like an evaporator (needs a drain) and air is recirculated within each cabin, allowing its temperature to be regulated by a local thermostat controlling a valve in the water supply. Like larger AirCon systems generally, chilled water systems usually contain several heat pumps. So there are reliability and efficiency benefits if one or more of these pumps are shut down once the system has cooled (or heated) the boat.

DC AirCon
Most AC powered AirCon systems draw too much power (particularly during compressor start-up) to be run routinely from batteries via an ordinary inverter. However, small air conditioning packages are available that incorporate a dedicated inverter and are designed to run from DC. Unless the boat has a DC generator set, the idea is that the unit runs via a battery charger while connected to shore power, and the boat’s engine-driven 12 Volt alternator if AirCon is wanted while under way. For many boats, this would cover most situations. At anchor, one could still run the main engine, or perhaps even a 12Volt portable generator on deck, although that would be inefficient and probably unacceptably noisy at night. Running the unit from batteries, even just to cool one sleeping cabin overnight, would require a considerable battery bank and a suitably robust recharging system.

Evaporative cooling
Most of these units, typically sold for RV use, do not have a compressor or contain any refrigerant. They cool the air simply by blowing it through a wet filter element, or by spraying a water mist into the air stream. The heat needed to evaporate the water comes from the air, which in the process becomes cooler, but also more humid. Evaporative coolers work very well in hot dry climates, but are far less effective where the relative humidity is already high, as it typically is near the sea.

Dehumidifiers
Another type of compressor based AirCon temporarily chills the air to remove moisture and returns it to the cabin at ambient temperature, but lower humidity. Portable dehumidifiers are often used to dry out (and lighten) racing yachts. Dehumidification for comfort might be more widely considered for yachts in the tropics, since it requires far less energy to run than conventional AirCon.

Capacity requirements
AirCon takes a lot of power, especially if the aim is to cool the boat during the heat of the day rather than just make sleeping conditions more comfortable. Cooling can be expressed as kW, but BTU/hour remains the most common unit in the air conditioning industry, especially in American equipment, and helps to distinguish electrical power input (in kW) from cooling or heating effect output (BTU/hr). A very rough calculation of the capacity requirements can be based on the cabin volume, allowing 500 BTU/hr per cubic meter (height x width x length). Thus a saloon/galley area of 4m x 3.5m, averaging 2m high would need a system rated at 14,000 BTU/hour. This is within the capacity of one fairly large water-cooled package system, but two might be needed to simultaneously condition the whole of a 40-50 foot yacht. AirCon isn’t compatible with open hatches, so natural ventilation can’t contribute but awnings and reflective films or shade covers can still help by reducing the heat load. Cabins with large glass areas may need significantly more cooling. Also, careful calculations should be done before purchasing a system, based on what you actually want to achieve, and manufacturer’s calculations.

Powering
Power availability may well be the limiting factor in selecting an AirCon system because when you want it most, in a hot and humid season, it will be running hard for long hours. For reliability, the continuously available power must safely exceed the maximum rating specified for the AirCon system. Even this rating, however, doesn’t cover the surge of power required to actually start the compressor which may be several times greater. This can prove a limiting factor, especially when powered from an on-board genset. Gensets and even marina shore power can also experience significant voltage sag when heavily loaded which reduces motor efficiency and increases the current draw. The motor may eventually overheat and cut out on thermal overload, or trip a supply circuit breaker.

Yachts with AirCon generally already have fairly sophisticated AC installations, but it’s definitely worth having an AC voltage meter in the boat’s power board, and shedding loads if the voltage drops below 215V.

One potential advantage of having several smaller package systems rather than one larger system is that the starting loads can be staggered, and that at least some cooling may be available when the power supply is limited.

It should also be remembered that cooling a whole cabin may be overkill, if just a cool breeze blowing over a bunk is all that’s really needed. Similarly, a system that in hot conditions can only really handle a single cabin, may, with open doors and a circulating fan, take the edge off the discomfort throughout the boat at night.