Refrigeration System Components

There are five key components to build a refrigeration system:

• Compressor - to compress the refrigerant gas (supplying the energy to pump the heat)
• Condenser - to condense the gas into a liquid (rejecting heat to surroundings)
• Evaporator Plate - where the refrigerant liquid changes state to a gas (absorbing heat from surroundings)
• Thermostat - switches the compressor on and off to maintain a relatively constant temperature
• Insulation - controls the rate of heat loss from the refrigeration enclosure

Many approaches have been taken to mobile refrigeration systems over the years. The biggest breakthrough was the advent of the Danfoss DC powered compressor in conjunction with efficient 12/24 volt DC energy systems.

It was once required to use an engine driven compressor that would store energy in a holding plate type evaporator to chill the refrigerated enclosure. The compressor would be either belt driven directly from an engine or powered by an AC motor that required a generator set. It would need to be run for around two hours per day—once in the morning and again in the evening. They are relatively high in cost, heavy and require a person to start an engine at the time dictated by the system.

The Danfoss system runs directly from stored energy in the battery bank. The compressor will run when it needs to. They use a light weight evaporator plate that has little impact on enclosure volume. A system that is properly manufactured and configured provides excellent service at significantly less cost compared to a holding plate system.

Selection of the right Danfoss system depends on box heat load and the preferred type of condensation method.

Condensor Set

The compressor is located between the evaporator plate and the condenser. For the condenser to change the state of the refrigerant gas to a liquid, the gas from the evaporator plate must be compressed to a level that causes condensation in the condenser. The energy required to compress the gas comes from the compressor motor which draws power from the battery system. The pressure required is directly proportional to the condenser temperature. Higher pressure causes more work for the compressor which is why a more efficient condenser will save energy.

The condenser takes the high pressure vapor from the compressor and cools the gas to the point where it changes state to a liquid. In other words, energy taken from the refrigeration box is transferred to the water or air in contact with the condenser. The choice is predominately air cooled or water cooled. Water cooled comes in two variants—a pumped water system or keel-cooled system.

In marine and land mobile application, the compressor and condenser are typically mounted together as an assembly called a condenser set. They are precharged with gas and ready to connect to an evaporator plate.  Each enclosure has a dedicated condenser set and evaporator plate. Condenser sets have capacity ratings expressed in BTU’s per hour. The ratings will vary according to evaporator plate temperature, condensing temperature and compressor RPM.

Please refer to the Refrigeration Cycle Wiki for a good overview of how a refrigeration system works.

Air Cooled Condensor Set
An air-cooled condenser cools the compressed refrigerant gas by use of a fan that blows ambient air across a finned condenser. They are the obvious choice for land based systems. An advantage for boats is their inherent simplicity and their ability to maintain refrigeration when the boat is dry. It is important to ensure cool air flow through the condenser—look for shrouded condensers with a plenum for connection of an air duct to direct the heated air away from the area where the condenser set is located.

Air Cooled Condensor Set System

Water Cooled Condensor Set
For vessels with larger box loads, water-cooling offers higher capacity and better efficiency. A DC powered pump transfers "cool" water through a tube-in-tube condenser. The water will take on heat as it passes through the condenser - getting warmer in the process.  Up to three condenser sets can be driven by one common filter and pump. System capacity is up to 30% higher than air-cooled compressors.

Water can be used from many sources. The most common source for boats is sea water however a filter will be required to protect the pump from debris. Water can be directly circulated from a drinking water tank provided that the design will limit the temperature rise of water in the tank.

Water Cooled Condensor Set System

Multiple systems from one pump

Keel Cooled Condensor Set
Keel-cooling is a unique form of water-cooled system. Instead of the condenser being mounted at the compressor and cooled by pumped water, the keel-cooler is in direct contact with the sea water. It is the most reliable of all systems. The only moving part is the compressor. It has similar performance to conventional water-cooled systems without the maintenance overhead of filter cleaning.

Keel Cooled Condensor Set System

Evaporator Plate

The evaporator plate takes liquid refrigerant from the condenser. Through an evaporation process, the liquid expands to a gas and absorbs heat energy from the plate. The ability of the evaporator plate to absorb energy from the enclosure depends on plate temperature and surface area. A plate of higher surface area is more efficient because the resulting plate temperature required to meet the target box temperature allows the system to work with a better coefficient of performance.

An ideal evaporator plate size will allow the plate temperature to be just below 0°C so that the condensate on the plate remains frozen, unless of course dripping condensate off the plate is not a problem. If the plate is spaced away from the wall of the enclosure, effective surface area can be doubled. Another way of increasing performance is to pass flowing air across the plate. We call this a fan forced evaporator. Note that this type of evaporator is not suitable for freezer applications.

Thermostat

A thermostat regulates the temperature of the evaporator plate by controlling when the compressor switches on or off so that the plate temperature is maintained near a desired setpoint temperature. Both mechanical and electronical units are available.

Insulation

This is the most overlooked element for a good refrigeration system. There are three considerations for insulation:

• Quality - closed cell foam will not allow moisture to condense on the inside of the box liner. Open cell foam will eventually draw in moisture which renders the insulation virtually useless.
• Thickness - The thicker the insulation, the less energy required to maintain temperature.
• Surface Area - Smaller boxes require less energy.