A simple look a pleasure boat refrigeration

[Richard Kollmann]

Marine mobile refrigeration engineering 101 on the basics. For example all liquids when evaporated absorb heat and the degree of heat absorbed depends on that liquid’s boiling point. Even a drop of water on your arm with air moving over it will cause it to boil cooling that spot on arm. It is not the vapor that absorbs heat it is the liquid during phase change that absorbs heat.

The job of the heat absorbing refrigerator no matter the evaporator device is in the liquid refrigerant volume and pressure provided by compressor/condensing flow interring evaporator with its very low pressure created by compressor that lowers box temperature. The efficiency of defining what occurs inside evaporators is verified by measuring changes of temperature and pressure inside evaporator referred to Super Heat.

Most companies selling marine refrigeration focus on additional box insulation to complement the type refrigeration they sell. I believe the refrigeration system along with boat’s energy power grid should determine the size and type refrigeration needed.

For our pleasure boat typical refrigeration there are only three commonly used Refrigerant Flow Control Devices.

!. Capillary Tube Refrigerant Flow Control
When it comes to mobile marine refrigeration the only advantage of a Cap tube refrigerant control is, it is less expensive. The disadvantages are the Cap tube limits the maximum amount of refrigerant that can be put in a system. These Cap Tubes meters refrigerant to evaporator but it has no moving parts to influence the super heat phase change inside evaporator. Mobile refrigeration especially in boats has to be able to compensate for varying ambient climate temperatures. It is not possible on Cap Tube systems to change evaporator super heat without adding or removing refrigerant.
A single Cap Tube should not be used on two or more evaporator as super heat in more than one plate is not adjustable. Holdover Eutectic plates with Cap Tube refrigerant flow control are very slow to freeze the first time and are recommended only on thermostat cycling eutectic plates and not 12 to 24 hour holdover plates.

2. Thermo Expansion Valves (TXV) Refrigerant Flow Control.
This valve is used to control the amount of refrigerant sent into the evaporator providing differential pressure between high liquid pressure and low pressure inside evaporator. This allows the vapor saturated mist to remove heat from the entire evaporator and not wasting energy on line returning to compressor. This full area inside evaporator is where temperature is continually dropping as heat is removed is defined as Super Heat absorbing intended area.
The internally pressure equalized TXV for mobile refrigeration would normally have a wide temperature range to allow operation with ambient temperatures from 104 degrees degree F and as low as 33 degrees F. The Btu or Tonage of TXV wide open orifice should be equal to output of compressor’s maximum output as defined by manufacturers installation specification sheet., too large an orifice can cause valve hunting and energy storing plates compressor problems. The TXV in addition to controlling refrigerant flow, super heat control, and it is adjusted automatically, by its temperature sense capillary tube attached to line leaving evaporator.

My conclusion agrees with others that a valve that repeats constantly stabilizes the TXV insures correct amount of refrigerant regardless of changes in ambient climate conditions is the best choice for mobile refrigeration. Another advantage with TXV’s is they permit adjustments to super heat without tampering with refrigerant volumes.

3. The Low Pressure Regulator (LPR) refrigerant flow control is not as popular as Capillary Tube or TXV controllers.
The low pressure regulator valves have been in use in soft drink machines for many years where ice is frozen in a low temperature area of machine and chilled soft drink in another area. When dispensing begins a drinking cup falls in place then ice drops and soft drink.
In the US marine industry SeaFrost has used the LPR to control small compressor refrigeration more than 30 years successfully. This valve is common on dual box refrigerator and freezer installed in Australia. The compressor supplies refrigerant to TXV in freezer and at the same time by use of a T in liquid pressure line the refrigerator box LPR valve that is manually adjusted to desired refrigerator temperatures.
The advantage the LPR valve it is preset to produce a fixed low pressure in evaporator.

[bdgWesternMass]

Hello Richard,

Thank you for putting this together. I will definitely use this as reference to frame my future questions.

I have already found existence of CO2 TXVs which where I will probably start my design.

Have a good day,

Ben

[longjonsilver]

Thanks Richard. Good write up.