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Wednesday, February 4, 2004

RV Absorption Refrigeration Basics


While the typical residential refrigerator is more like an air conditioner since a mechanical compressor is utilized, the RV absorption refrigerator remains a mystery to many. It is probably the most perplexing of all the LP-burning appliances found in the typical RV today. Understanding the theory of absorption and “absorbing” why it takes heat in order to make cold can be mysterious even for the most engineer-like amongst us. The usual result is to simply shake our head and grab another cool beverage from the fridge. But by grasping a basic view of the theory of absorption and understanding the importance of leveling, ventilation, and cleanliness, the RV refrigerator can provide many years of RVing enjoyment and independence. Let’s take a look at the absorption refrigerator by first examining the heart of the refrigerator, the cooling unit.



A typical RV refrigerator cooling unit consists of four major components; the boiler, condenser, evaporators and absorber. The unit is sealed and pressurized and is non-field-repairable. The internal contents of the sealed absorption system include water, liquefied ammonia, hydrogen gas and sodium chromate, the chemical used to line the internal pipes to protect them from the corrosiveness of the ammonia.

During a typical cycle, heat is applied to the boiler by either an electric heating element or an open LP flame. Water and liquid ammonia begin to boil and ammonia vapor is passed on to the condenser. The water is separated and returns to the boiler section. After condensing back into a liquid, hydrogen vapor is introduced to the liquid ammonia at the low temperature evaporator located in the freezer compartment. In short, heat from inside the freezer is transferred through the pipes of the low temperature evaporator and released to the exterior. Since the absence of heat is cold, the freezer and everything in it becomes cold.

Next, a remnant of mixed hydrogen vapor and liquid ammonia passes to the high temperature evaporator located in the main food storage compartment and the process of removing the heat continues albeit, less assertively. The food storage compartment/high temperature evaporator becomes cold, but not as cold as the low temperature evaporator in the freezer.

Upon leaving the evaporator sections, the weakened ammonia liquid then flows into the absorber coils. Here the hydrogen vapor rises back up to the evaporator section while the ammonia is mixed with water in the absorber vessel, flows to the boiler and the process starts all over again.

From the point where liquid ammonia leaves the condenser until it reaches the absorber and re-mixes with the water, gravity provides the impetus. Off level operation slows the pace and causes overheating in the boiler section. Continued operation in an over-heated condition results in cooling unit blockage when the sodium chromate particles turn to crystal and block sections of internal piping in the boiler. Unfortunately, in RV absorption refrigeration, this process cannot be reversed. The only remedy for a blocked cooling unit is replacement. The lesson – take the time to properly level the RV as demanded by the principles of absorption refrigeration. The bottom line: keep it level, keep it ventilated, keep it clean. Your refrigerator and your wallet will thank you.

But how do we do that, you ask? By applying basic preventive maintenance practices, that’s how. That term, “preventive maintenance,” is probably familiar to your already if you’ve owned an RV for more than fifteen minutes. I’ve said this before; the wisdom of the concept behind the phrase is often recognized long after the fact. Such is often the case regarding the absorption refrigerator. All too often RVers obtain their education the hard way – through trial and error, and mostly by error at that.

The sophisticated systems found in RVs today can be intimidating for the do-it-yourself type unless he or she is equipped with the proper tools, time, and attitude. Also, the incorporation of printed circuitry, electronically controlled gas valves, 120-volts AC, 12-volts DC and LP fuel in one appliance can be somewhat puzzling unless you possess a degree of understanding regarding the operation of such an appliance. Therefore, performing your own RV refrigerator clean and service will require more than just a mechanical inclination; it will also be beneficial to possess an understanding of the operation of your particular unit as well as of the theory of absorption refrigeration. In addition, the do-it-yourselfer must always place safety as the number one priority. With this in mind, always be sure that the LP container has been completely shut off and that all forms of electricity have been disconnected before performing maintenance on any LP-burning appliances. And for the feint of heart, remember, every RV service facility stands at the ready to perform an absorption refrigeration service for you. As a reminder, here are a few items that need annual attention on any absorption refrigerator:

1.  Burner assembly and orifice. Must be accessed, disassembled, cleaned and reassembled.

2.  Flue and flue baffle. If yours is accessible, remove it and clean away the carbon deposits. A specifically designed flue brush may be necessary.

3.  Ignitor assembly. Likewise, the spark probe, ground probe and flame sensor must be free from carbon deposits.

4.  Cooling unit. The cooling unit is accessible through the exterior refrigerator side vent. Vacuum and clean all exposed components and tubing. Inspect and clean the condenser fins at the very top of the cooling unit. Remove any blockages.

5.  Refrigerator receptacle. Check for proper polarity and voltage at the refrigerator receptacle located at the rear of the refrigerator.

6.  Printed circuit board contacts. Clean and preserve the contact strip on the printed circuit board and all other electrical terminals.

7.  Main LP regulator. Be sure it is adjusted to 11.0 inches of water column and that the RV is LP leak-free.

8.  Storage/non-use. After the camping season ends, be sure to thoroughly clean the inside of the refrigerator completely. Remove all food and wash down all plastic components. Do not use an abrasive cleanser. Block the refrigerator doors open to allow free flow air to circulate inside.

Congratulations! You have, at least, gained an insight into the importance of preventive maintenance, as well as completing RV Refrigeration 101. For the details of how to actually perform an absorption refrigerator clean and service, check out this article.


Cleaning & Servicing the RV Absorption Refrigerator


The common term, "preventive maintenance," is applicable in any number of industries today - the RV industry included. Unfortunately, the wisdom of the concept behind the phrase is often recognized after the fact - for instance, when it comes time to repair, or even replace, the absorption refrigerator in a recreation vehicle. All too often RVers obtain their education the hard way - through trial and error, and mostly by error at that. How many RVers have been known to exclaim, "I sure didn't realize that refrigerators for RVs were that expensive," or "You mean there is something that has to be maintained on an RV refrigerator?"


Contrary to the belief that since absorption refrigerators have no moving parts they require neither attention nor service is the very real fact that many RVers are paying for repairs that could have been avoided had "preventive maintenance" been employed.

Most RVers understand that roof seams, caps, vents and antennas must be sealed periodically to keep the moisture out. It is also common knowledge that RV holding tanks must be flushed out occasionally to prevent blockages. Likewise, all propane gas appliances must be serviced periodically in order to avoid costly repairs and to ensure that they operate at peak performance throughout the RVing season. It is likely that the absorption refrigerator is the most visible, most used and yet, the least understood of the appliances.

For most RVers, the fact that absorption refrigerators require heat to produce cold is baffling enough in and of itself. This article is not intended as a treatise regarding the principles of absorption refrigeration, (refer to this article for that info), rather, it is intended as a discussion of the technical methods used by RV service organizations today in performing the maintenance task traditionally known as the refrigerator "clean and service." In many cases, astute coach owners who possess a bit of technical know-how can perform some of these tasks.

The sophisticated systems found in RVs today can be intimidating for the do-it-yourself type unless he or she is equipped with the proper tools, time, and attitude. Also, the incorporation of printed circuitry, electronically controlled gas valves, 120-volts AC, 12-volts DC and propane fuel in one appliance can be somewhat puzzling unless you possess a degree of understanding regarding the operation of such an appliance. Therefore, performing your own RV refrigerator clean and service will require more than just a mechanical inclination; it will also be beneficial to possess an understanding of the operation of your particular unit as well as of the theory of absorption refrigeration. In addition, the do-it-yourselfer must always make safety the number one priority. With this in mind, always be sure that the propane tank has been completely shut off before performing maintenance on any propane-burning appliances.

What tools will be necessary? Aside from the normal assortment of basic hand tools (those every RVer should carry anyway), several specialty tools will be required to perform a thorough clean and service on an RV refrigerator. These special tools include a thermocouple tester, a polarity tester, a volt-ohm meter, and a flue brush, all of which are relatively inexpensive items that can be ordered or purchased locally. Once obtained, these tools could theoretically pay for themselves within a short time.

A manometer will be used by a professional RV service technician to measure the line pressure of the propane gas system. A vapor pressure between 10 and 14 inches of water column must be maintained in order for every propane appliance designed for use in a recreation vehicle to function at its maximum efficiency level. The manometer is used to monitor the regulator setting to insure that the pressure is set correctly. The industry standard for the pressure is 11 inches of water column (W.C.). Some RVers may assume pressure is measured in pounds per square inch (PSI), but the pressure required by propane system on any RV is so low that a smaller unit of measurement must be used. Translated, 11 inches of water column is equal to .4 PSI. In other words, it's the amount of pressure required to raise a vertical column of water 11 inches high. A standard u-tube manometer is 100 percent accurate since it is an actual tube filled with water and marked off in increments of inches.

The thermocouple tester is used to check the effectiveness of the thermocouple on the refrigerator, if it indeed has one. Most modern RV refrigerators will not be equipped with a thermocouple. The tester can also be used in testing any propane appliances that relies on pilot ignition. This tester can be ordered through any reputable RV parts store or service facility.

The polarity tester is useful prior to plugging any coach into shore power. The tester will assure that the campground wiring is correct. (Of course, this is assuming that the coach wiring is correct to begin with.) The polarity tester simply plugs into any 120-volt AC receptacle inside the RV. For the purposes of this article, it should be plugged into the same receptacle as the refrigerator. A series of three lights (light-emitting diodes, or LEDs) will indicate whether the circuit is correctly wired; if it is not, the tester will identify exactly how it is miswired.

A small, inexpensive volt-ohm meter should be included in every RVer's tool kit. The multimeter has so many uses that it is not possible to list them all here. For the purposes of this article, the device is used to measure the AC voltage at the refrigerator receptacle and also to check the battery voltage at the fridge. These tests take only minutes but could eliminate the frustration of downtime during a trip.

The last specialty tool that is necessary is a flue brush. The flue brush is used to clean the flue assembly of the boiler portion of the cooling unit at the rear of the refrigerator. This brush can be obtained from most automotive supply stores and parts houses. Although not likely labeled specifically as a "flue" brush, it is a round, cylindrical brush with a long wire handle. It has a diameter of approximately three-quarter inches. Unlike the other specialty tools, this one will be applicable only for servicing the refrigerator.

How much time will it take to perform the refrigerator clean and service? Allow two to three hours to do a thorough job. Be advised, however, that this process may take longer with some refrigerators, especially when performed the very first time. But this is usually mandated by the manner in which the refrigerator has been installed.

As a general rule, for correctly installed refrigerators, it's likely to take between two and five hours. The refrigerator should be cleaned and serviced at least once each RVing season, as should the other propane appliances - the furnace, the water heater, and the range. If exposed to dusty or sandy destinations, or if stored in such an area, it may be necessary to clean and service the refrigerator more often. At times, lack of use can be more harmful to propane appliances than abuse.

The three most common causes of propane-related problems with correctly operated absorption refrigerators are dirt, (see photo above), spider webs and inactivity. Notice the stipulation "correctly operated." We are all aware that correct leveling and efficient ventilation are essential to the proper performance of the refrigerator. An entire series of problems could develop when these areas are neglected, but that's a different story altogether.

So, how do you perform a clean and service? With tools in hand and a positive "can do" attitude, are you ready to begin? Not quite. Now is the time to study the operator's manual, user's guide, parts breakdown, and/or service manual for your particular brand and model refrigerator. Try to familiarize yourself with as much literature pertaining to your particular unit as possible. One of the biggest mistakes that even many professional RV technicians make is rushing into a job without enough product knowledge to support their efforts. Don't make the same mistake. When you are confident that you are able to recognize and locate such components as flue baffles, burner orifices, and heating elements, and when the propane container has been completely shut off, then you can proceed.

Cleaning and servicing your refrigerator will involve some degree of disassembly and/or partial removal of the unit, which will usually be mandated by the type of factory installation. For instance, if the gray water holding tank pipe extends upward through the roof and is situated against the burner box at the rear of your refrigerator - which happens - then it will be necessary to partially remove the refrigerator to gain access to the components. If it is necessary to partially remove the unit, disconnect the flare fitting for the gas line at the rear of the fridge and remove all of the screws that secure the unit in the compartment.

At this time, one would be wise to install a flare plug in the gas line, for two reasons - as a safety measure, in case someone inadvertently turns the gas back on, and also to keep dirt and dust from entering the open end of the gas line. Ordinarily, a 3/8-inch flare plug will be required. You'll also likely need to remove the mounting screws that attach the fridge to the cabinet. Once the refrigerator is loosened, it can be slid into the coach hallway by pushing from the outside. Slide the unit only far enough to allow for access to the burner area components. If the burner area is readily accessible without moving the refrigerator, the time required to complete the job will be greatly reduced. Most of the time, the latter will be the case.

The burner assembly should be completely removed for inspection and cleaning. Remove the burner orifice and allow it to soak in carburetor cleaner, acetone, or a like solution. Set it aside to air dry. Inspect the burner carefully. Look for cracks or warping, especially at or near the primary inlet holes if the burner is so equipped. Overheating caused some earlier model burners to warp, thus interfering with the normal process of mixing air with LP and resulting in improper combustion. This particular phenomenon has been all but eliminated by virtue of newer unit design.

After the orifice has dried, inspect the it carefully, preferably through a magnifying glass. The fact that the orifice has been soaked and air dried does not guarantee that it is clean and free from obstructions. The spider webs that are commonly found in burner orifices can be incredibly strong and stubborn. Their presence will be detected only with the aid of a magnifying glass. UNDER NO CIRCUMSTANCES should you insert any object - drill bits, broaches, paper clips, needles, etc. - into the orifice opening. Because these orifices are made primarily of brass or aluminum, (some older refrigerators had orifices made of rubies!) they are fairly soft, and the opening could easily be distorted. Were this to happen, the Btu rating and gas flow rates would be affected, and unusual flame characteristics could develop. If the orifice cannot be cleaned, it should be replaced with one of identical size or with an identical part number. Should you determine that you need to replace the orifice, it is recommended that you consult a professional RV technician.

The spiral flue baffle also must be removed and cleaned. It is situated directly above the burner and is located inside the flue tube. The baffle is installed from the top of the refrigerator, and the method used to secure it will vary from model to model. You will notice that the baffle is suspended by a wire to a predetermined height. This baffle height is critical to your particular refrigerator, and care should be taken not to bend or otherwise alter this dimension.

Once the baffle is removed, the next step is to run the flue brush through the empty flue tube to dislodge any carbon deposits and rust that might be clinging to the side of the tube. These deposits could hinder the flue in its task of evenly distributing heat and drawing the flame into the tube. This step can usually be accomplished through the roof vent, or by partially removing the refrigerator as discussed earlier. In some instances, especially when working with coaches that have curved sidewalls, it might be necessary to remove the entire refrigerator.

Before performing the next step, you must determine which type of unit you have. If your refrigerator is one of the electronic ignition types with an automatic reignite feature then it will be necessary to clean both the electrode and the thermocouple, which are situated directly above the burner and positioned in the flame. This electrode is the flame sense probe, which through normal use can become coated with a deposit of carbon, thus preventing the printed circuit board from doing its job. Clean the probe and the thermocouple by brushing them lightly with a fine-grade steel wool pad. When properly cleaned the probe and thermocouple will appear brighter.

If your refrigerator has an older, thermo-electric safety valve, just a thermocouple will be positioned in the flame. This thermocouple is designed for use with the flame failure safety valve. Should the flame be blown out accidentally, the flow of LP will stop. It is a good idea to clean the tip of this thermocouple, as described above, to insure proper operation. When heated, the thermocouple actually produces voltage that energizes an electromagnet situated in the safety valve. As mentioned earlier, this style of flame failure safety device is not found in modern RV refrigerators.

At this point you should check the thermocouple if your unit has one. Insert the thermocouple into the tester. Heat the tip of the end of the thermocouple that you just brightened by holding it in a flame, and make sure that it holds the magnet of the tester. Thermocouples do fatigue and can weaken; so, if you doubt its effectiveness, replace it.

The next step is not so technical, but it is equally important, as it can have a direct effect on the proper ventilation of the cooling unit. Clean the rear of the refrigerator through the access door on the outside of the coach and possibly through the roof vent. You can either blow the area clean with compressed air or vacuum away the dust and dirt. At this time, conduct an overall visual inspection of the cooling unit area, paying special attention to the condenser fins situated at the uppermost portion of the cooling unit. If the roof vent opening is not protected by a screen, birds and other critters have a tendency to construct their nests on the condenser fins where it is nice and warm. Even when a screen protects the vent, birds will still make their nests up there where it's nice and warm (see photo). This will restrict the airflow over the condenser, causing an overheating situation that could eventually ruin the cooling unit.

In addition to the LP components, one must also check the electrical operation of the refrigerator. Now that you have the refrigerator partially removed and the rear of the unit clean, insert the polarity tester into the 120-volt AC receptacle used by the refrigerator. Plug the coach into shore power to determine whether the polarity is correct at the receptacle. If the tester indicates anything other than the correct polarity it will be necessary to isolate and correct the discrepancy. Disconnect the shoreline and contact a qualified electrician or technician to troubleshoot the problem; unless you have adequate training for dealing with alternating current, please leave the troubleshooting to a qualified individual. You can also check for proper voltage at this receptacle by using the volt-ohm meter. Likewise, measure the amount of battery voltage present at the electrical connection at the rear of the unit.

Servicing the new refrigerators that utilize printed circuit boards involves an added maintenance step. First you will have to gain access to the board where it connects to the multi-plug. Remove the connector and clean the board contacts with a common pencil eraser. This will remove any corrosion or residue that could possibly cause a poor connection and result in erratic performance and improper cooling. (Editor's note: some experts are of the opinion that if the circuit board if working properly it should be left alone). This line of thinking would agree with the old maxim "If it isn't broke, don't fix it."

It is now time to reassemble everything. Insert the flue baffle into the flue tube, and make sure that the flue extension is secured properly if so equipped. Reassemble the burner orifice, the burner tube, and any other portion of the burner area that you removed, doing so in the reverse order in which you disassembled them. If the refrigerator was partially or completely removed during the procedure, now is the time to reinstall it. Be sure to replace all of the screws that were removed. Reconnect the gas line (which will usually necessitate the use of two wrenches), taking care not to twist any portion except the flare nut itself. Now mix up a solution of dish detergent and water. (Do not use detergent that contains ammonia, however, as it has a detrimental effect on copper tubing.) Now, turn the valve on at the propane container and brush or spray this soapy solution onto the flare connection at the back of the unit to check for leaks. Before you go on to the next step, determine that no leaks exist.

The final step in completing your refrigerator clean and service will benefit all of your LP-burning appliances. The LP pressure regulator must be set at 11 inches of water column. (Review the section at the beginning of this article concerning manometers.) It is recommended to take the RV to a pro shop to have a Certified RV service technician perform a timed, pressure drop test to insure no LP leaks exist anywhere in the RV.

Review. Let's review the primary steps performed during a refrigerator clean and service. Doing so will enable you to double-check your work and to insure its completeness without sacrificing the safety factor. If you have any doubts at any time during the procedure, stop and review. Consult the literature to confirm the location of any of the components mentioned for your particular brand and model.

Step 1: Completely turn off the LP at the tank.
Step 2: Partially or totally remove the refrigerator as necessary to gain access to the components.
Step 3: Remove, disassemble, and clean the burner assembly and orifice. Blow dry with compressed air.
Step 4: Remove the flue baffle. Clean the baffle and the flue tube while the burner is removed.
Step 5: Clean and brighten the thermocouple and the flame sense probe or ignitor probe, depending on how your unit is equipped.
Step 6: Test the thermocouple if so equipped.
Step 7: Inspect and clean the rear of the cooing unit, especially the condenser fins.
Step 8: Check for the proper polarity of the 120-volt AC receptacle used for the refrigerator.
Step 9: If so equipped, clean the printed circuit board contacts using a pencil eraser.
Step 10: Reassemble and reinstall all components and the refrigerator itself.
Step 11: Turn on the LP tank and check for leaks.
Step 12: Have the LP pressure set to 11 inches water column.

The final test will be to light the refrigerator in the propane mode and observe the burner flame as it is pulled into the bottom of the flue tube. The flame should be a bright blue with a possible tinge of orange at the very top. It should also be centered directly under the bottom of the flue tube.

Congratulations. You have successfully completed a professional refrigerator clean and service. Hopefully you have gained an insight into the importance of preventive maintenance, as well as the satisfaction of knowing that your refrigerator will be operating at peak performance. And remember your refrigerator will last longer if this periodic maintenance procedure is performed every RVing season.

Disclaimer:

In all instances, every effort is made to ensure the correctness of all content on the RV Doctor Website. It is imperative that if you choose to follow any instructions or procedures outlined on any page of this website, you must first satisfy yourself thoroughly that neither personal nor product safety will be compromised or jeopardized.

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If you are in doubt or do not feel comfortable about a procedure, do not continue. Simply call your local RV service facility and make an appointment with them. The advice, recommendations and procedures offered by the RV Doctor are solely those of Gary. They do not necessarily reflect the opinions, procedures and recommendations of our sponsors or advertisers.