Author Topic: Design Your Own efficient Icebox Refrigeration Part #1  (Read 393 times)

Richard

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Design Your Own efficient Icebox Refrigeration Part #1
« on: March 04, 2018, 03:10:34 pm »
In most cases when you are considering refrigeration for your boat you become the design engineer and application engineer. Icebox conversion refrigeration systems are not all the same and there are no two boat installation application the same. There are a dozen brands of conversion units offering over one hundred options to select from. Boat show refrigeration displays sometimes even with operating units provide a general idea of what a system looks like. When communicating with a refrigeration brand salesman general installation information is given by someone who has not looked inside your boat and may not really understands the degree of refrigeration you are looking for.

After installation 50% of the actual refrigerator performance is a result of system design. The other 50% is how unit is installed in your boat. Advantage of visually seeing the compressor condensing unit allows you to determine if it is compatible with your boat�s design. The principle of refrigeration is to remove heat from cool area inside a box and exhaust heat to another warmer area. On most boats under 36 feet the compressor condensing unit will be installed in a cabinet or under a seat and not in a large open well ventilated area. For the compressor condenser to function efficiency it must be capable of drawing in ambient boat air temperatures and forcing warmed process air to another area. Fan cooled horizontal air flow condensers are usually recommended. Keep in mind the length of refrigerant lines between condensing unit and evaporator will vary with each manufactures conversion brand.

No longer does one icebox conversion refrigeration system work for all temperature conditions and all box sizes.  Understanding component selection when buying or building an ice box conversion refrigeration system is important. When selecting a refrigeration system knowing the size and shape of a box must come first. The average home refrigerator today is 16 to 20 cubic feet is separated into cooler, refrigerator, and freezer temperature zones. Not all foods benefit from the same stored temperatures so when considering an icebox conversion the type of evaporator and its location is important. Box insulation is also important to avoid box exterior moisture condensation while operated at the lowest box desired interior operating temperatures. Moisture on the insulation�s exterior indicates the amount of insulation is not sufficient.

 When considering 12/24 volt refrigeration for boats under 36 feet the average daily amperage and Btu per cubic ft of refrigerated space needs to be determined. Converted space of a refrigerator with insulation R value of 20 at ambient temperature of 70 degree F will require 5 amp-hrs per cubic foot a day. Increase the insulation to R30 value in tropical weather of 90+ degrees F the daily amperage will be increased to approximately 10 amp-hrs per cu ft per day. These power estimates allow for a twenty degree F freezing area using 20% of box area, If lower freezer temperatures are required additional daily amperage and Btu�s will be required.. Yes, these estimates exceed boating industry advertised figures as they represent the average blue water tropical climate cruising boater experiences.

There are four main components of an ice box conversion refrigeration system; compressor, evaporator, condenser and refrigerant flow control device. The volume of heat per hour in worst case removed from each cubic foot of refrigerated box to maintain recommended standard refrigerated box temperatures might be 600 Btu and 1200 Btu for each cubic foot of freezer volume. Worst case refrigerator designs apply to boats operating in latitudes below 25 degrees. Boats in cooler temperature climates averaging 70 degrees F can cut my figures 2% per each degree ambient day temperatures cooler than 90 degrees F. Example 70 degree average climate temp versus worst case 95 degree amperages and Btu projections are reduced by 50%.

Compressor
After determining Refrigerated box in worst case daily Btu requirement and maximum amperage selection of compressor�s size is easy.  When using the Danfoss 12/24 volt older fixed speed fixed Btu compressors BD2.5 and BD3 one size had to fit all sizes of refrigerated boxes. If compressor was too small or too large for a fixed speed Danfoss it was energy inefficient. The newer variable speed BD compressors can be matched to the box�s daily heat removal load requirement by selecting the most energy efficient compressor speed. All three Danfoss variable speed compressors are Btu rated at various speeds and evaporator temperatures. I believe a fair comparison of each refrigerator�s performance is when, evaporator temperatures are maintained at +12 degree F.
 Btu output and best Coefficient Of Performance (COP) of each compressor are available from Danfoss:

These hourly figures are believed to represent a good comparison with an evaporator temperature of Plus 10 degree F (- 12 degrees C) and a condensing gas temperature of 131 degrees F (55 Degrees C) 

BD35 at 2000 rpm 214 Btu and COP of 1.4 and 3.1 amps.
BD35 at 3500 rpm 349 Btu and COP of  1.28 and 5.38 amps.

BD50 at 2000 rpm 273 Btu and COP of 1,31 or 1.74 and 4.18 amps.
BD50 at 3500 rpm 535 Btu and COP of 1.23 or 1.64and  7.8 amps.

BD80 at 2500 rpm 471 Btu and COP of 1,66 and 6.9 amps.
BD80 at 4400 rpm 936 Btu and COP of 1.51 and 12 amps.

Note These are ASHRAE  Engineering Charts


The higher the ratio of COP energy consumed by compressor the more efficient it will be. To realize the total Systems Coefficient of Performance (SCOP) the other three major components must be in close equilibrium balance to compressor�s output. Size of all refrigerant piping (copper tubing) through out the refrigeration system will be  established by the size of the suction refrigerant return line fitting at the compressor. Danfoss BD compressor return gas fitting can be either Metric or Standard for � inch outside diameter copper tubing. Some systems may use 3/16 OD tubing for high pressure side of a BD compressor system. A Filter Dryer is required in the liquid high pressure line prior to refrigerant flow control device to collect any lose materials and absorb existing moisture left after assembling of complete system. Over size filter dryers are recommended for DIY systems.
There are other 12/24 volt refrigeration similar capacity compressors to the Danfoss, Cubigel, Sikelan,Colku, Boyard, Sanheng and Retelc, although they are only used by one or two boat refrigeration companies. Should on site maintenance support  or parts be required in other world ports the Danfoss compressor is a good choice.
Condenser
There are five types of refrigerant condensers in pleasure boat refrigeration when compressor size is less than 1000 Btu:
�   Static air: Older and small refrigerators use open air natural convection air over vertical wire and tube condensers. One manufacturer of an icebox conversion unit recommends that the compartment where this condensing unit is located should be at least 100 cubic feet in order to dispose of process heat efficiently. Adding a convection air fan or blower to force air to circulate over condenser coils will greatly improve condenser efficiency. For more information on performance improvements see FAN section on my web site.
�   Convection fan: For refrigeration units with capacity of less than 1000 Btu the most efficient and reliable condenser for all climate conditions is a forced air shrouded fan with a cooling fin condenser. The secrets to fan cooled condensers is their design efficiency and size. Correct installing of condenser in boat must be to insure that warmed exiting heated air is not allowed to pass through condenser again.   
�   Circulating water: Any type of a water cooling system on a small refrigeration system will compromise refrigerator reliability for a number of possible reasons. High risk exposures due to unfriendly refrigerator down time with water as a condenser cooling medium are; short life of water circulating pumps, regular water filter maintenance , marine growth build up in water piping, water flow restricted due to air bubbles entering condenser water loop. The most severe risk is catastrophic failure to refrigeration system due to low voltage discharge corrosion that occurs in water cooled condensers.
�   Submerged condenser outside hull in water:  Keel coolers or refrigerant lines in seawater outside boat where seawater medium�s temperature cannot be controlled is only going to be efficient in mild seawater temperatures. The disadvantages of these systems are problems of electrolysis corrosion and regular exterior cleaning requirements. When operated in warm seawater, compressor overheating can cause very expensive refrigerant flow problems and eventually total system failure.
�   Use of fresh water tank to dispose of process heat: Two marine refrigeration companies using small BD compressors have recommended an additional water cooled condenser circulating water by pump from fresh water tank to reduce some of the very troublesome water cooling problems. Using fresh water tank water to when air cooled condenser design fall short can result in a new set of problems.
The advantages of correctly installed air cooled condensing units are generally more reliable than any type of water cooling. Correctly installed air cooled condensing units draw air from one area and exhaust warm heated process air to another open area. If the unit you are considering is not designed to dump heated air to another area it is not going to perform well in warm climates.
There are two major disadvantages of using BD compressor, condenser, water cooled, refrigeration systems currently in use; lack of compressor metal mass cooling from normal condenser fan and loss of control module�s compressor overheat protection safety circuit. Preventing hermetic compressor overheating has always been a problem for refrigeration designs on pleasure boats. BD size compressors are fully hermetic where electric rotation field coils and armature are covered with oil making heat removal important. Most air cooled designs must rely on refrigerant flow to dispose of all process heat. It is also assumed that compressor�s external heat can be removed by moving air across it with the existing condenser fan. Attaching a small fan to compressor was tried by one system manufacturer of boat systems. Another company used a water tank attached to a � horse power water cooled system�s compressor in an attempt to reduce process heat.
The basic design practice for refrigeration system designs is to protect compressor from failure by limiting high liquid pressure and high compressor temperature. There is usually a high refrigerant pressure switch or temperature amperage overload device mounted in or on this compressor to protect it. The BD compressors rely on the electronics in control module to protect compressor from damage. If a BD variable speed compressor cooling fan amperage exceeds � amp module believes the condenser is overheating and signals the safety circuit in module to stop compressor from running. Water cooled BD compressor systems do not use the module safety circuits connected to terminals Small + and F.  This fan circuit is designed to protect against damaging high pressure/temperature by fan compressor mass metal heat reduced.
If a compressor manufacturer wanted their compressor�s to be water cooled there would be an application engineering data sheet produced by Danfoss defining water cooled operating limits.
Maintenance costs of water cooled BD compressor systems can cost as much as five times the cost of a well designed air cooled unit. Actual in service life of water cooled units is unpredictable. Maintenance of an air cooled BD compressor refrigeration unit is much the same as a home refrigerator such as cleaning dust and hair out of condenser coil inlet frequently and replacing the cooling fan in 5 to 10 years.
Evaporator         
The evaporator in a system is where refrigerant is sprayed into a gaseous form of vapor. As the saturated mist converts to vapor it absorbs heat from evaporator plate and refrigerated space. Size, type and shape of evaporator depend on the box size that is intended to be refrigerated. Each system manufacturer provides more than one size evaporator to match each compressor�s rated Btu capacity.
�   Bare Tube Evaporator: Bare tube evaporators are not normally found in icebox conversion kits but can be used on do it yourself systems. Bare tube evaporators are sometimes used to replace a failed evaporator plate that can not be removed like one installed behind box�s liner.
�   Plate type evaporators: By attaching at least 25 feet of copper tubing coils on a metal plate is  one method used to fabricate an evaporator. As long as an  evaporator coil is in full contact with plate�s surface thermo heat transfer is more efficient than just a copper coil by itself.                             
�   Roll Bond Evaporators: Laminated metal evaporators are the most efficient and very popular with all small refrigeration companies. The roll bonding process of two or more layers of metal squeezed together under high pressure acts as a cold welding permute process. Before heavy roll bonding a special material is used on plates where refrigerant flow channels in the evaporator plate are to be formed. After bonding is complete high controlled nitrogen pressure forced between plate layers will expand refrigerant passage channels. 
�   Finned  Evaporators: are tube evaporator coils with metal fins spaced along the tubing. The surface area volume of each fin increases heat transfer from the box moving heated air closer to refrigerant. If fin type evaporators are used in a refrigerator, fin temperatures during compressor off cycles must remain above freezing. If fin coil evaporators are used at refrigerator temperature below 33 degrees F they will develop sufficient air blocking frost and become very inefficient as they are not self defrosting.
�   Eutectic Evaporators:  A eutectic evaporator plate is different from conventional plates in that by freezing a eutectic solution inside or other phase change material large amounts of energy can be stored. The larger the volume of eutectic solution the more Btu of energy can be stored inside eutectic plates. By mixing water and a type of antifreeze to establish a freeze point phase change will occur at that temperature. Different percentages of solutions can be mixed for refrigerator and freezer plates.  The ratio of frozen water to antifreeze determines the cooling effect as it melts.  When comparing different frozen solution mixes in the same volume plates Btu energy stored there is the same. A freezer plate melts twice as fast as a refrigerator plate so it needs to be twice the size of refrigerator plate. I believe that the only way to justify a holding plate on a small BD compressor refrigeration system is if there is alternative surplus energy available from solar or wind.                                 
Refrigerant Flow Control Devices
Refrigerant control devices meter flow of refrigerant between the high pressure and low pressure sides of a system. They also maintain a pressure difference to allow the high pressure fluid to flow into the low pressure evaporator where it vaporizes and absorbs unwanted heat. There are three types of flow control devices used with these small refrigerator compressors, Capillary Tube, Thermostatic Expansion Valves and Low Pressure Regulating Valves.
�   Capillary Tube Refrigerant Flow Control: With small refrigeration units having standard evaporators the capillary tube metering device with its carefully sized flow tube restriction will extend compressor life. Capillary tube refrigerant flow control prevents high pressure Btu overloads and also extreme low velocity cooling air and lubricating oil return to compressor. The length of capillary tube and its correct inside diameter will drop refrigerant pressure to maintain desired Btu cooling capacity range. The control strategy for small boat refrigeration is to keep it simple and efficient. Capillary Tube refrigerant controls are maintenance free unlike the valve controls with moving parts. For any of the refrigerant flow devices to be reliable there must be a filter or screen just prior to fluid intering the flow device orifice.  With a warm evaporator, there is fluctuating liquid and gas vapor interring Cap tube that changes mass flow as it passes through the long tube. With the variation of flow as evaporator cools refrigerant lines can vibrate and the hissing sound heard in the evaporator will change as final gas condensing to liquid occurs. The main disadvantage of a capillary tube flow control is in mobile marine refrigeration where ambient air temperatures are subject to changes that affect volume of refrigerant charge. Example correct refrigerant charge in New York�s ambient air temperatures can result in an over charge in a tropical climate like Miami. Manufacturers of ice box conversion kits service condensing unit and evaporator separately with correct refrigerant charge by weight in grams.
�   Thermostatic Expansion Valve  (TXV):  The thermostatic valve maintains a constant flow of high pressure liquid into evaporator�s much lower pressure where the refrigerant mist changes to a gas vapor absorbing heat. The TEV Flow control metering inside evaporator is regulated by a temperature sense bulb mounted outside evaporator on return refrigerant gas vapor line going to compressor.  The temperature sense bulb when warm reduces spring pressure inside TXV opening flow through evaporator. As the temp bulb cools TXV closes slowly keeping the heat absorbing process ( Super Heat) inside evaporator as much as possible. The disadvantage with TXV flow control on very small system is modification expense over a basic system without an appreciable performance improvement. The main concern with TXV used on a small compressor system is finding a valve that matches compressor Btu output without exceeding design limits of compressor. It is true that TXV valves with small refrigerant storage tank receivers used on Danfoss BD compressor eutectic holding plate systems can reduce daily energy use.
�   Low Pressure Regulating Vales: Low pressure Evaporator Pressure Regulator (EPR) controls refrigerant flow and has been used by SeaFrost for at least twenty five years successfully on small Btu compressors. The EPR valve used on SeaFrost units is installed in the high pressure line with an external evaporator pressure adjustment knob. Unlike other flow control devices that continue to lower evaporator temperature/pressures as long as compressor runs the EPR once adjusted maintains a fixed evaporator temperature/pressure. The normal design purpose for EPR valves is to control different evaporator temperatures in the same refrigerator system. A 134a refrigerator evaporator operates its best with a low pressure of 6 to 10 psi and a freezer evaporator from 3 to 6 psi. When the compressor is large enough a TXV control would be used on freezer evaporator and a EPR control used for refrigerator plate, unfortunately Danfoss BD compressors are too small for two evaporators in parallel. My dislike of using EPR valves on ice box conversion units is the refrigerant low side pressure drop in each installation is different so adjusting EPR valve sometimes takes several days to find the correct valve setting.
Suction Line Accumulator