Keeping Your Cool With A/C
Part 2Whats new with refrigerants and controls.
Last month, in Part 1 we described basic types of air-conditioning systtems, principles of operation, and how to size a system to your boat.
Refrigerants: R-12 Is Gone, R-22 Will Follow
All manufacturers of marine air-conditioning (A/C) systems must comply with the Montreal Protocol of 1987, an international agreement to protect the earth’s environment by reducing the emission of ozone-depleting substances into the atmosphere. Products such as R-12, a chlorofluorocarbon (CFC) that was formerly the refrigerant of choice for automobile and many marine A/C systems, have not been used for some time. Present-day systems primarily use hydrochlorofluorocarbon (HCFC) refrigerants such as R-22. Even though of less potential harm than the CFCs, the HCFCs must be phased out by 2030 except in so-called developing countries, which may continue to manufacture and use them until 2040. The US EPA has mandated the phase-out of HCFCs by 2010. In the usual bureaucratic foot race to be first, the European Union has set a phase-out date of 2009.
Replacements for HCFC refrigerants are already available and are being used in some of the larger marine A/C systems.
AquaAir is building systems using R-407C refrigerant, which, with its zero ozone depletion potential (ODP), will remain acceptable into the future. The buyer of a direct expansion or chilled water marine A/C system need not be overly concerned with the type of refrigerant used. Most of these systems run for their entire lifetimes without requiring any service to the refrigerant loop. In addition, numerous substitute refrigerants have been developed that can be used to service systems that are now filled with obsolete refrigerants.
A/C System Controls
Improvements in the refrigeration cycle have evolved slowly, primarily driven by the needs of markets other than marine air-conditioning. At the same time, improvements in marine A/C control systems are impressive.
The basic marine A/C system control has long consisted of a rotary three-position switch: “Off,” “Start” and “Run,” plus a rotary knob-controlled thermostat devoid of any meaningful temperature markings. Some systems add a third rotary knob used to slightly slow the speed of the system’s shaded pole blower motor.
The two- or three-knob systems are attractive only in their simplicity. They have a number of annoying shortcomings. They are unable to deal with momentary power interruptions, a common occurrence when plugged into shore power and inevitable when switching from shore power to an on-board genset or vice versa. Restoring power to an operating A/C system in less than about a minute after an interruption creates a “locked rotor” situation in the system’s compressor. The high refrigerant pressure created under normal operation is still present, preventing the compressor drive motor from turning. The excessive current flow that results generally causes one or more circuit breakers to trip.
On systems equipped with reverse-cycle heating, a slight movement of the thermostat can inadvertently switch operation from cooling to heating, often tripping the circuit breaker. With manual controls the seawater pump usually runs continually, regardless of whether the compressor is in operation.
Microprocessor monitoring of numerous operating parameters does a great deal to protect the A/C system. Restoration of power after a brief outage is dealt with by automatic cycling of the refrigerant flow-reversing valve. This reduces the internal pressure to zero before restoring power to the compressor motor—no more circuit breaker trips. The same technique is used when switching from cooling to heating mode. Microprocessor controls provide well-calibrated digital temperature readout of cabin temperature and set point and often include exterior temperature sensing that allows the system to anticipate any required increase or decrease in cooling that may result from changes in the outdoor environment.
Dehumidification cycles may be programmed to occur automatically even when the vessel is unoccupied.
Controls allow a selected temperature to be maintained, operating the system in either the heating or cooling mode as necessary.
A useful attribute of a digital control is its ability to progressively reduce blower speed as the temperature approaches the set point and then cycle the compressor off and on to maintain the desired temperature.
Digital controls also provide superior performance in protecting the system from damage from overvoltage and most importantly when using dockside power, undervoltage. Operating an electric motor from too low a voltage can cause overheating and failure. Digital controls help protect the system in the event seawater flow is interrupted.
The similar appearance of many digital controls is because most are built by one company. An exception is the Cruisair SMX series, which was one of the original electronic controls and continues to provide performance equal to or in some areas superior to anything else on the market. The location of the temperature sensor is one area of difference. Most controls incorporate the temperature sensor in the faceplate of the control panel. Sunlight falling on the faceplate of the A/C control can cause a cabin temperature sensing error. The SMX uses a remote temperature sensor placed in the return air flow to the A/C unit.
The Passport II controls used by a number of manufacturers can be programmed to operate the air circulation fan at increasing speeds as the cabin temperature approaches the set point when a reverse-cycle system is in the heating mode. This provision can prevent nuisance shut-downs that might occur under low air flow conditions with other types of controls.
Overall, we prefer the Cruisair SMX system to all other electronic control systems. Some boat owners will place special value on the system’s ability to work with a modem, allowing control of the system via telephone. Cruisair also plans to introduce a wireless remote control.
Keeping It Working
Today’s marine A/C systems require little routine maintenance other than removal of trapped dust from filters and cleaning of the seawater strainer. The air filter on the evaporator should be removed and washed clean or replaced. A vacuum cleaner equipped with a soft pick-up brush is used to remove accumulated dust from the evaporator surfaces. Periodically clean the condensate collection drip pan and its drain hose. Periodically inspect the flow of seawater being discharged from the hull-side fitting. A photo taken of the discharge when the system is new will serve as a useful comparison tool. Remember to keep the seawater strainer clean. In areas where A/C use is constant, the metal seawater strainer basket may erode very rapidly. Fortunately, Groco has recognized this problem and can supply special erosion-resistant plastic baskets for many of their strainers.
The A/C system may be the highest electrical power consumer on a boat. Periodic examination of the wiring terminals can head off problems caused by corrosion of the wires or loosening of mechanical connections. In some areas, particularly in warm, southern waters, the sea water system can rapidly become clogged with marine growth. Although water-cooled condensers are generally resistant to the growth of marine organisms, fouling can occur. Should this happen, the condenser can be cleaned by an air-conditioning service company which will circulate a special cleaning/descaling fluid through the condenser.
Marine A/C System Options
While self-contained A/C units are the most common types, remote condensing units (often called split systems) are often used in larger vessels where space for the condensing unit (which contains the compressor) is available in the engine compartment. A single condensing unit can supply a number of air handlers, each containing an evaporator and blower. These systems resemble home A/C installations, with two refrigerant pipes connecting the condensing unit (the outdoor unit in a home system) to the evaporator/fan assemblies located in the occupied space. The advantages of the remote condensing system include less noise in the cabin and the relatively small size of the air handler. The disadvantages include somewhat higher equipment cost and the need to install, connect, evacuate, charge and leak test the refrigerant piping system. Remote condensing systems are available with capacities from 5,000 to 60,000 Btu/hour (0.4 to 5.0 tons).
Reverse-cycle heating is not generally used in these systems. Cabin heating is typically accomplished with electrical resistance heat in each air handler.
Glacier Bay, best known for its electric motor-powered holding plate refrigeration systems, offers a system that uses the refrigeration compressor to operate the A/C evaporator at times when refrigeration cooling is not required. A 12V-powered 1-hp. compressor, the “Whisper Jet,” provides 5,900 Btu of air-conditioning capacity while consuming an average of 55 amps. Glacier also offers larger capacity systems. Installation of these systems is similar to the conventional split A/C systems, requiring field installation of refrigerant lines, evacuation and charging.
Ocean Breeze offers an interesting alternative approach to self-contained A/C units called the Whipseraire series. These systems are designed to be installed in a space remote from the accommodation, including an engine room. The unit is completely enclosed, isolating it from the surrounding air. A high velocity (and higher-than-normal power consumption) blower forces cooled air through small diameter PVC pipes to air distribution outlets in the cabin. The manufacturer claims that a couple of 2" diameter pipes are sufficient for air delivery, with a 6" diameter pipe used to return air to the A/C unit.
Because virtually all of today’s marine A/C systems use common refrigeration system components there is not a great deal of material difference on which to base the selection of one unit versus others offering equal features. The buying decision can, of course, be based on the price of the basic chassis and the control panel. However, unless you plan to install the system yourself you will want to obtain a detailed quote for the total installation. It is important to consider the reputation of the installing company as well as the reputation of the manufacturer.
Be sure to obtain, carefully read and compare the written warranty that applies to the systems you are considering. For example, Cruisair and Marine Air warrant their direct expansion units and mechanical controls for two years, their electronic controls for three years and will pay for removal and reinstallation during the first year. Some other manufacturers warrant the compressor for up to five years, excluding coverage for items such as the water pump or control. Ocean Breeze quotes a five-year warranty for the compressor, evaporator, condenser and fan motor. Mermaid states their warranty is five years for parts and labor, except for one year for the water pump and two years for the thermostat. We believe that equipment such as direct expansion marine A/C systems are, to a degree, subject to “infant mortality,” an increased risk of failure during the first few months of use, after which a long period of trouble-free operation can be expected. For this reason, the remove-and-reinstall warranty coverage offered by Cruisair and Marine Air can be very attractive.
Use care when comparing manufacturers’ specifications. Systems using electric heat will usually be less costly than those offering a reverse-cycle heating mode. Even when heating is not a major requirement, as in Florida, the presence of the reversing valve, operated by an electronic, microprocessor control, will be valuable in ensuring that internal refrigerant system pressure is reduced to zero before each start. This feature will be best appreciated when a momentary power interruption occurs. A system without the ability to bleed off trapped pressure will likely trip its power circuit breaker, either the one in the boat or the one on shore. Resetting it at 0200 will be no joy.
When comparing unit cooling capacity check the manufacturer’s claimed cooling Btu ratings versus the running current, including the power required for the air blower. Because compressors of the same type are largely identical, claims of lower power drain per Btu should be viewed with suspicion.
Contacts- Aqua Air, 1050 E 9th St., Hialeah, FL 33010, 800/457-3928. AquaCal Corp. 2737 24th St N., St. Petersburg, FL 33713. Cruisair and MarineAir, Taylor Made Environmental, Inc., PO Box 15299, Richmond, VA 23227-0699, 804/746-1313, 804/746-7248 fax. HFL Generators, 1951 N.E. 54th St., Fort Lauderdale, FL 33308, 954/938-5200, 954/938-5200 fax. Mermaid Marine Air, 2651 Park Windsor Dr., Unit 203, Fort Myers, FL 33901, 800/330-3553, 941/4188-0535, 941/418-0538 fax. Ocean Breeze, Quorum Marine & Electronics, Inc., 1305 SE Dixie Highway, Bay A, Stuart, FL 34994, 407/220-0038. Westerbeke, Avon Industrial Park, PO Box 181, Avon, MA 02322, 508/559-9323, 508/559-9323 fax.