Marine AC Compressors: Keeping Boats and Ships Comfortable

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Replaced a defective Cruisair A/C condensing unit and evaporator unit with CTM CX Condensing Unit and EV Evaporator Unit. Was so happy with the CTM systems I replaced four other working Cruisair systems. Very easy swap out. CTM Condensing units very quiet. Had a minor defect in one of the new units and it was replaced same day after easy warranty claim.

Ensuring a pleasant climate on board a vessel is crucial for the well-being and satisfaction of both passengers and crew. Whether you're navigating a sailboat through the Bahamas, embarking on a trawler for the Great Loop voyage, or operating a sportfishing yacht in pursuit of big game in the Florida Keys, the importance of a comfortable environment remains paramount. The same applies to commercial vessels and boats engaged in charter operations; unpleasant conditions such as excessive heat, humidity, and stale odors within cabins, galleys, salons, and other interior spaces can significantly diminish the overall experience. 

The key to keeping interior spaces of all types of boats comfortable is having a quality air conditioning system that's engineered for the unique demands of marine climates. There's a lot to consider when choosing the best system for your size and type of vessel, as well as the type of climate and environment where most of you will be boating. 

When it comes to marine A/C, there are two key factors to consider: performance and energy efficiency. But, even knowing this, navigating through the details can be a bit tricky due to all the marketing jargon. No need to stress, though; here are some key details to keep you in the loop:

Unit Capacity 

Marine air conditioning systems have a unit capacity rating based on two measurements: BTU/h (British Thermal Units) and kW (kilowatts), where 1 kW is equivalent to BTU/h. The air output capacity is assessed by two values at a specific outflow. One of these values is Sensible Capacity, which gauges the temperature drop of the air. The other is Latent Capacity, representing the amount of condensate (water) removed from the air. When combined, Sensible Capacity and Latent Capacity form the Total Capacity for an air conditioning system.

Here is where things can get confusing for the boater. Not all boat air conditioner manufacturers provide their capacity/rating information in a straightforward way like Dometic Marine. For example, some manufacturers only compare output air temperatures. This is misleading, as running one unit at a lower fan speed will demonstrate a colder output air temperature vs. another unit at a higher fan speed. This does not reflect a complete picture of the system’s cooling capacity. The only true way to measure a unit’s capacity is in a calibrated laboratory setting.

Factors Affecting Performance & Energy Efficiency 

The performance and efficiency of A/C units can be influenced by various environmental factors, with sea water temperature playing a significant role. As sea water temperatures rise, the performance of A/C units tends to decrease. Warmer water makes it more challenging to expel heat, resulting in increased power consumption for the A/C unit.

Sea water flow can also greatly affect A/C unit efficiency; lower or restricted sea water flow rates can cause increased power consumption. Therefore, maintaining clean and unobstructed sea strainers is essential for boats to ensure optimal A/C unit performance and accurate power consumption measurements. 
  
Generator voltage is also important when assessing A/C power consumption. Generators may not consistently output the same voltage, especially when operating multiple loads on a vessel. A decrease in generator voltage can result in an increased amp draw on the air conditioner. For this reason, it’s important to always consider both the current and voltage for accurate power consumption measurements. 
  
All of this leads to the central point of this article: It’s important for both boaters and boat builders to fully understand marine air conditioning. With accurate information, it becomes possible to make meaningful and fair comparisons, cutting through conflicting marketing claims.  
  
For example, Dometic bases all its capacity measurements on a sea water temperature of 85o F (29.5o C). When evaluating any A/C unit, make sure the manufacturer specifies the sea water temperature they use to benchmark their performance. This can make a significant difference in a unit’s reported capacity — and is a strong indicator of how it will perform in real-world conditions. 
  
Some Dometic Marine competitors have made marketing comparisons against our units, claiming that they use less current. This can be misleading, as less current draw might simply mean the unit is underperforming in capacity. This means that it's not necessarily more efficient in a practical sense that matters to boaters. 
 
Several component features play a role in determining the efficiency and performance of marine air conditioning systems. Heat exchanger size is one such factor, influencing an A/C unit's capacity based on the surface area. Caution is advised when encountering small units boasting higher capacities, as physics suggests that a smaller heat exchanger may not outperform a larger one. 
 
Valve modulation is another important consideration. While some air conditioners use fixed capillary tubes for refrigerant flow, others employ thermal expansion valves or electronic expansion valves that adapt to changing environmental conditions for improved performance.