Author Topic: Refrigeration Design Part #2  (Read 685 times)

Richard

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Refrigeration Design Part #2
« on: March 04, 2018, 03:11:56 pm »
All mechanical designs of a refrigerant system must address possible excessive liquid refrigerant and oil returning to compressor. There must always be some assurance that accumulation of liquid will not reach and damage compressor pistons. Large systems have suction line accumulator containers to capture liquids and then slowly return them in vapor form to compressor. Small systems with Roll bond evaporators have circular traps near the end of refrigerant�s flow through an evaporator that serve as, liquid control accumulator traps. Most icebox conversion units incorporate longer than needed refrigerant lines between condensing unit and evaporator that allows liquid refrigerant to return to vapor eliminating need for a discharge accumulator. Not all evaporators are designed with suction line accumulators so if return line is less than 10 feet there is a possibility of liquid slugging compressor.
Control and Monitoring Additions
I have defined a reliable basic icebox conversion unit parts list Kit it will  still require a compressor control module, thermostat, compressor speed control and condenser fan.
�   Compressor Electronic Control Module:  There are several modules types available for the Danfoss variable speed 12/24 volt BD compressors.
Standard 12/24 volt electronic module:101N0210 was improved and replaced with new 101 N0212 module.
AC/DC module: design when power supply can be either or both AC/DC. This module is recommended if boat is to spend time connected to ships generator or shore electrical power.
EMI: Electro Magnetic Interference module recommended to reduce radio noise when refrigerator is running.
AEO Adaptive Energy Optimization module: AEO changes compressor speed automatically to find most energy efficient speed while maintaining preset desired box temperature. Highly recommended for boats where increased compressor speed at times is not fast enough to maintain desired box temperature.
�   Thermostat Temperature Control: There are four basic types of mechanical thermostats used in pleasure boat refrigeration: Zone or area thermostats for coolers and spillover boxes, Refrigerator, Freezer and full range temperature controller with adjustable differential. Electronic thermostats are not recommended in pleasure boat marine refrigeration applications due to their reliability.

Cooler and Spillover Thermostats:  Require a thermostat with its temperature sense tube  in free air space and not touching holding plate or evaporator. Use a +6 degrees F differential and temperature thermostat range from +27 to +50 degree F. This is also a good spillover thermostat range for controlling temperatures in refrigerator side of the spillover box.

Refrigerator Temperature Thermostat Controlling Evaporator Temperature: This Snap action standard manual thermostat control evaporator with a  temperature range from -13 F to +32 degrees F is recommended. Differential will range from 8 degrees, at dial setting, with the thermostat set to max cold

 Freezer temperature thermostat: This recommended thermostat for icebox conversions with standard evaporator temperature control. use a temperature range thermostat of  -26F to +5F with a Differential of  20 to 13 degrees F.
 In many cruising boats it is desirable to have a full range temperature box so it can be used at times as a freezer or refrigerator or even a drink cooler. The best full range mechanical thermostats are listed as Temperature controllers with an adjustable range of  -15 to +40 degrees F and an adjustable differential from 5 to 40 degrees.

�   Box Temperature Thermometer: With the exception of drink coolers checking temperature inside boxes with remote thermometers is desirable. Electronic temperature gauges are inexpensive but generally have a limited life on a boat. Mechanical temperature thermometers can be calibrated to perform well but are limited to where they can be installed.   
�   Condenser Cooling Fan : Condenser process heat disposal and refrigerant liquid high pressure are controlled by condenser fans efficiency. Fan amperage if monitored by compressor control module to prevent compressor overheat damage. If module detects high amperage above � amp module will prevent compressor from running. Ambient air interring compressor is normally considered to be standard day 70 degrees F so a kit manufacturer might for sound level purposes use a 35 Cfm rated 12 volt 4.68 inch muffin fan. This size fan is used to cool many types of computer electronics. They are available in ratings from 35 to 130 Cubic Feet per Minute (CFM). Air from condenser fan is also required to help remove heat from compressor. If condensing compressor unit is to be located in a warm engine compartment or when it will spend time in a tropical climate conditions a larger noisier fan is recommended. Fans that produce 80 and higher cfm can not be considered if amperage exceeds � amp due to module high amperage temperature protection. Danfoss BD variable speed compressor will run on 12 or 24 volts but the fans will always be powered by module at 12 volts.
�   Electronic Speed Control: Can be fixed by a single resistor in thermostat wire or automatically with electronic plug in circuit boards. Selecting the correct compressor speed will result in an energy efficient system.
�    Built in electronic troubleshooting LED:  A trouble analyzer Light Emitting Diode (LED) costing $2 is not included in many Danfoss refrigerator kits. It is important to install trouble LED even it may never light up. If this starts flashing a code every it is telling you why compressor is not running.