Refrigerator Chassis

Fig 1 Refrigerator with freezer section on top

Take the refrigeration components, controls, and ancillary devices such as ice-makers, fans, lights, and water valves out of a refrigerator and one is left with a plastic lined compartment surrounded by insulation and and a pretty outer covering of thin sheet-metal (okay, very thin sheet-metal on most new refrigerators). An insulated box with the least amount of insulation that the market will accept. That amount seems to be about two inches these days which works out to about R10. Put your hand on the side of a refrigerator when it is running and feel the cold. Then try to guess who is paying for that cold. If you try this on a freezer or dorm fridge with the condenser beneath the sheet-metal skin, you feel heat rather than cold.

The person that decided to make an R-10 insulated box and put the condenser just outside of the insulation (so that heat leaks back into the interior of the box through the inadequate insulation) was the same one that decided that putting a hot compressor under the refrigerator also made sense. Heat rises so while the compressor runs to make your box cold, it is adding to the heat-load that it must extract. I hear someone screaming my refrigerator is energy-star rated and is much more energy efficient than those old heavy (well built) energy-hogs of the past. I am saving the planet you mean man. Most of the reduction in energy usage has been the result of better insulation (not more of it) and the down-sizing of compressors, evaporators and condensers. Under-sizing systems to save energy may be why refrigerators now last eight years instead of twenty to fifty.

The modern refrigerators is a poorly designed appliance and none of the major manufacturers seem interested in rethinking the issue. One would believe that some of them were in the electric utility business.

Filter-dryers and Capillary Tubes

Replacement filter-dryer

Filter-dryers are placed at the outlet of the condenser in household refrigerators and contain mesh screens to trap contaminants and chemicals to absorb moisture. This provides protection to the capillary tube which can become clogged and block the flow of liquid refrigerant to the evaporator. A completely blocked capillary tube will stop all refrigerant from reaching the evaporator and no cooling takes place.

A clogged capillary tube is difficult to diagnose since it seems the unit is low on refrigerant (the evaporator doesn’t receive refrigerant, but not because it has leaked out). This is one situation where piercing-valves and manifold gauges are necessary to be certain. A system with a clogged cap tube and a good compressor will pull the low-side suction pressure down into a vacuum as low as 20 inches of hg. The condenser will be cold since it contains liquid refrigerant under high pressure. If the high-side pressure is low and the low-side is in a vacuum, the problem is likely a lack of refrigerant rather than a clogged capillary tube.

The filter-dryers installed to protect the capillary tube can also clog and the symptoms look the same as a clogged capillary tube. If a filter-dryer is only partially clogged and creates a pressure drop it will be coated with frost. A frosted dryer or frost on the cap tube at the outlet of the dryer indicates a partially clogged dryer that needs to be replaced. Dryers are cheap, but the economics of hiring a refrigeration tech to recover the refrigerant, replace the dryer, and then evacuate and recharge the system will depend on the age and cost of the system and type of refrigerant. R12 systems are likely better retired.

Capillary tubes usually clog in the first few inches after the dryer unless the system is contaminated with moisture. Moisture will almost always freeze and form an ice-plug at the inlet to the evaporator. This happens if the system is opened, moist air allowed to enter the piping, and a complete evacuation with a quality vacuum-pump is not performed prior to recharging.

Cap tubes can be replaced, but it is not an easy task. Almost all are attached to the suction line for much of their length. Their internal bore and length are part of the refrigerator design and cannot be replaced with just any cap tube one might have. The size depends on the type of refrigerant and capacity of the compressor.

In my foolish youth, I used to replace clogged cap tubes on almost new refrigerators by removing the evaporator, suction line, and cap tube back to the compressor area (as long as they were not buried in the foam insulation) and replacing the whole thing with a similar evaporator/suction line/ cap tube assembly from a scrapped refrigerator. The refrigerator that I am using now received this treatment, a new compressor, and a charge of R-409a over eight years ago.

In the past year I have seen three Frigidaire side-by-side refrigerators in which the capillary tube had not been brazed into the filter-dryer during manufacture. Thus they were sold (and returned to the retailer) without refrigerant. The replacement of the filter-dryer, evacuation, and charging with R-134a was worth it since they retailed for almost two thousand dollars each. Two other new Frigidaire units that I saw had broken cap tubes where they entered the filter-dryer. This likely happened in shipment (its a long way to China) due to the crappy placement of filter-dryers on Frigidaire side-by-sides.

Simple Ways To Know If Your Fridge Is Low On Freon

Technicians who work on central air conditioners and heat pumps use manifold gauges to find the Freon pressure and indirectly the evaporator and condenser temperatures by connecting to access fittings on the unit. These gauges have hoses of various lengths and when they are removed from the access fittings some Freon is retained in the hoses. This is not a problem with systems that contain pounds of Freon since the amount retained in the gauge manifold hoses is a small percentage of the correct charge.

Household refrigerators and freezers usually do not have access fittings. This is not necessarily a bad thing. Some refrigerators work with as little as eight ounces of R-134. Connecting gauge manifold hoses to the high-side of the system could leave a large percentage of the charge in the hose when it is disconnected. To connect to the system and read Freon pressure one must use a piercing-valve, which clamps on the refrigerant tubing and punctures a hole to allow pressure readings. These valves must be left in place afterwards and provide a place for refrigerant to leak if they are not clamped tightly or if the seals deteriorate over time.

I only install piercing-valves on a refrigerator or freezer as a last resort. Despite the common belief that almost every refrigerator problem is due to the lack of Freon that is just not the case. Most refrigerator malfunctions are caused by the failure of electrical components. An insufficient Freon charge is one of the last things I consider when troubleshooting a refrigerator that does not cool. FREON LEAKS OUT; IT DOES NOT WEAR OUT. It is unlikely that your refrigerator will suddenly start leaking just sitting in your kitchen UNLESS it is manual defrost and you recently removed the ice from the freezer compartment by using a knife, screwdriver, chisel, or pry bar and punctured the aluminum evaporator coil or an incompetent service person installed piercing-valves that are leaking.

Conditions which may seem like lack of Freon but are not:

Evaporator fan not running which ices up the evaporator and raises freezer and fresh-food compartment temperature.

Condenser fan not running which raises temperature of freezer and fresh food compartment.

Dirty condenser which raises temperature of freezer and fresh food compartment.

Defrost problems (if automatic defrost) which ices up evaporator raising temperature in freezer and fresh food compartment.

Old compressor with worn valves that raises evaporator pressure and temperature.

Worn door seals that allow hot air to leak into the freezer or fresh food compartment (Note: Some Haier refrigerator doors won’t seal even when new.)

Clogged filter dryer or capillary tube which keeps the Freon from circulating properly.

All the above conditions make the compressor run continuously or almost continuously, but so does a lack of Freon. The compressor runs continuously because the refrigerator cannot reach the set-point on the cold-control. Keep in mind that if you set your cold control to mid-range and it can’t cool to that setting, turning it to max (which asks it to make the temperature even colder) does absolutely nothing.

The fastest and easiest way to find out if the unit has Freon is to turn it off and listen. When a compressor pumps Freon the evaporator pressure decreases and the condenser pressure increases (thus we have a high and low side of the system). When the compressor stops running the refrigerant pressures in the system slowly equalizes through the capillary tube. This can take several minutes. Turn the cold control to the off position and put your ear to the side of the unit. If you hear hissing and gurgling noises the unit has some Freon, which is equalizing through the capillary tube (it may not be a correct charge). If you hear nothing, the unit contains little or no Freon or has a clogged filter-dryer or cap tube OR the compressor is running but not pumping (worn or broken valve). None of these conditions are as likely as having a defective fan, dirty condenser, or defrost problem.

If you hear the hissing and gurgling of Freon, but the unit won’t freeze ice or the ice cream is soft, unplug the refrigerator and remove the cover from the evaporator. The evaporator is in the freezer compartment near the evaporator fan. Each model is different so spend a few minutes to figure out how to expose the evaporator. Some GE and Magic Chef models place the evaporator in the bottom of the freezer compartment which makes repairing the defrost system difficult. On an upright freezer the evaporator may be tubes that are part of the shelves so nothing needs to be removed. Chest freezers have the evaporator beneath the inner lining and usually cannot be exposed.

If you remove the panel covering the evaporator and find a big block of ice, you either have a defrost problem or the evaporator fan wasn’t running. Sometimes a defrost problem will form so much ice that the fan can’t run. Thaw the evaporator and see if the fan will run before condemning it.

If the evaporator is not encased in ice arrange the cover so that the unit can be restarted. This may be difficult if the fan is attached to the panel. The capillary tube is a small copper tube entering the evaporator. The suction line at the evaporator outlet will be much bigger.

Liquid refrigerant (freon) entering the evaporator begins to boil into a gas because of the sudden drop in pressure created by the compressor. This change of state absorbs heat. The temperature inside the evaporator coil may be -20 degrees F. This will condense and freeze moisture in the air in contact with the coil (which is why we have to defrost). If there is freon boiling inside the evaporator this frosting will occur within minutes. So if we run the unit with the cover off the evaporator and no frost forms we can know that there really is no freon or the the system is clogged. If the unit runs for several minutes with the door closed and the freon charge is correct, the frosting should form all along the length of the evaporator.

Other signs of the presence of freon is heat in the condenser. If the unit has been running for several minutes and the condenser is cold, there is either little or no freon or the system is clogged or the compressor is not pumping.

Haier Ye Haier Ye

I haven’t warned people lately that Haier refrigerators are CRAP!

http://fridgeman.wordpress.com/2008/01/04/most-new-refrigerators-are-crap/

http://www.consumeraffairs.com/homeowners/haier_refrigerators.html

Others seem to agree. If you are considering buying a Haier, please send me the money. Your food will be just as cold sitting in the kitchen floor and I’ll be better off. Besides, not buying a Haier saves land-fill space for important things like toxic waste.

The Refrigeration Cycle

Most homeowners do not have EPA certifications or the necessary equipment to handle, recover, or charge refrigerants so this post is intended for educational purposes and to give the non-technician a basic understanding of how domestic refrigerators use the refrigeration cycle to preserve food. I will not try to explain the chemistry of refrigerants, but will explain the basic principals and thermodynamic laws that cause a refrigerant to create cold conditions inside the unit.

Cold is a term of convenience, but cold actually doesn’t exist. Heat is a form of energy. Cold is the absence of heat, although until one reaches absolute-zero even that definition is incorrect. There is nothing wrong with pronouncing that the inside of our freezer is cold as long as we know that it is less hot than something else that we are using for comparison (usually the temperature of the room where the unit is located or our body temperature).

A colder substance or object always absorbs heat unless it is perfectly insulated. A cube of ice in our drink cools the liquid because melting ice (changing states from a solid to a liquid) absorbs a lot of heat from the surrounding liquid. When the ice is melted, our drink absorbs heat from the surrounding air and begins to warm again. Most people think that an ice cube cools the drink because it melts and mixes cold water into the drink. It is true that the melted cube leaves cold water in our drink but the majority of cooling is due to the cube changing states (solid to liquid) and absorbing energy from the surrounding liquid. If we could magically gather the water from our cube and refreeze it, it would give up the energy it absorbed when it changed from a solid to a liquid. The only way we can get the absorbed heat from our water is to put it somewhere colder than 32 degrees so that the heat energy will flow from the water to the colder surroundings and cause a change of state from liquid to solid. We do this when we freeze a tray of ice cubes.

Now that we know that it takes energy to change a solid to liquid, lets consider other substances besides water. Most metals are solids at room temperature. We have all seen pictures of furnaces used to melt steel. Changing steel to a liquid takes a lot of heat energy but it is useful to mold it to the shape we desire. As soon as the liquid steel is removed from the heat source of the furnace it gives up heat to the much colder surrounding air and becomes a solid again. All substances change states depending upon the temperature (and pressure). Some solids have to be very hot to change to a liquid. Some gases have to be cooled to extremely low temperatures to make them condense (change from a gas to a liquid). Liquid oxygen and liquid nitrogen are two examples.

Water boils at 212 degrees F at sea-level. Some refrigerants boil at -30 degrees F. It may seem odd that something cold enough to freeze our skin is boiling but as the refrigerant changes from a liquid to a gas, it is absorbing heat (even if that heat feels very cold to us). When liquid refrigerant enters the evaporator of a refrigerator or freezer, it absorbs heat as it changes into a gas (evaporates). Even if the freezer is cold (0 to 10 degrees F) it is hotter than the refrigerant so heat flows to the refrigerant. When the heat inside the freezer flows to the refrigerant, heat energy is removed from the air and contents of the freezer and it gets colder.

Fig 1 Four basic parts of a refrigeration system

Evaporators are designed so that liquid refrigerant evaporates (absorbing heat) and leaves the coil as a gas. The compressor pumps the gas out of the evaporator coil through the suction line. The compressor pumps the refrigerant gas into the condenser and raises the pressure from less than 10 psi to around 130 psi. This compresses the gas and raises its temperature (which is why condensers are hot). The refrigerant in the condenser gives up heat (absorbed from inside the refrigerator) to the surrounding air as it condenses into a liquid. Because the refrigerant is under higher pressure it condenses at a higher temperature. The liquid refrigerant is forced by the pressure of the compressor into the cap (capillary) tube for another trip to the evaporator. This cycle is constant as long as the compressor is running.

The cap tube is a small copper line with a precision internal bore measured in thousandths of an inch connecting the condenser to the evaporator. It is used in domestic freezers, refrigerators, and air conditioners. It meters liquid refrigerant into the evaporator and maintains a pressure differential between the high-side condenser and low-side evaporator.

This is a very simple and basic description of the refrigeration cycle but all mechanical refrigeration systems operate on these principles.

Defrost Timer

I am always being asked where the defrost timer is located on a refrigerator. Unfortunately the location varies depending on the manufacturer and model. Some are located in the rear near the compressor. Others are behind the louvered kick-plate in the front. Some are in the fresh-food section near the cold control. To make finding them more challenging they are usually covered except for a hole that allows them to be manually advanced. On the refrigerator that I use the timer is in the fresh-food section beside the cold-control. The only indication is the hole that permits a large flat blade screwdriver to be used to advance the timer.

Fig 1 Typical Defrost Timer

 

In a typical defrost circuit 120 volts is passed to the timer from the cold-control when it calls for cooling. The voltage energizes a clock circuit. If the defrost timer is in the defrost position, voltage is sent to the defrost heater circuit. If the timer is not in defrost, voltage is passed to the compressor starting-circuit and fans.

 Fig 2 Typical defrost circuit (defrost cycle)

Some defrost circuits power the clock with line-voltage (bypassing the cold-control). This arrangement initiates a defrost cycle at set intervals of time regardless of how often the compressor has run. In the circuit above the clock runs only when the cold-control is made so it initiates a defrost cycle based on accumulated compressor run-time. With the defrost timer in the defrost position current goes to the defrost heater. The thermostat in the circuit (also called a termination thermostat) opens to turn off the defrost heater when the temperature in the freezer reaches its design temperature. This is usually around 50 to 55 degrees. This keeps the freezer compartment from getting too hot before the defrost timer switches back to the compressor run position. If the thermostat fails in the open position no current can flow through the defrost circuit even though the defrost timer and defrost heater are good. In this case the evaporator will frost up and eventually the freezer and fresh-food compartment temperatures will rise. The same symptoms will occur if the clock in the defrost timer fails and never switches to a defrost cycle or if the defrost heater fails.

Fig 3 Typical defrost circuit (in compressor run position)

Normally the termination thermostat will turn off the defrost heater before the defrost timer times out and switches the defrost heater out of the circuit. When the timer times out the defrost timer opens the heater circuit and closes the contacts to send power to the compressor and fans. The defrost timer stays in this position until the accumulated time to switch back to defrost cycle.

Compressor Start Circuits II

Domestic refrigerators and freezers have single-phase compressors and are usually less than 1/2 horsepower. Small single-phase compressors have start and run windings. The purpose of the starting relay is to take the start winding out of the circuit when the compressor has reached its rated rpm. If the compressor fails to start, the problem could be internal to the compressor or in the external starting circuit. If the compressor tries to start but cannot because of a shorted or open winding, an open internal overload, or a seized bearing, one usually hears a hum followed by a clicking as the external overload in the starting circuit opens from over-current. Unfortunately, a defective starting relay can also cause the same hum and clicking. An open in the starting circuit relay or overload will prevent voltage from reaching the compressor windings and no hum or clicking will be heard since no current is flowing.

One other situation where a hum and/or clicking of the external overload will be heard (and a situation that is normal) is when a running compressor loses power and an immediate attempt is made to restart it. This might happen if the power is interrupted for a few seconds by a storm or other fault in the electrical grid. It usually takes three or four minutes for the compressor to restart under these circumstances and one may hear the clicking of the external overload more than once.

Even though compressors do fail, starting-circuits fail much more frequently. It may be worth the cost of a new relay and overload to prove that the compressor really is bad before buying a new refrigerator. Sometimes the defect in the starting circuit is obvious. Wires and terminals may be burnt or loose or parts may be melted or obviously broken.

WITH THE REFRIGERATOR UNPLUGGED inspect the wires to the compressor, remove the cover from the starting relay/overload and look for damage such as broken or loose wires or burnt connections. Gently pry the relay/overload off the compressor terminals. Shake the relay. If it sounds like a miniature baby-rattle, bad things have probably happened inside it. If it looks melted or smells burnt, it is likely damaged. A few times I have found refrigerators with good relays but defective (open) overloads. If the overload is a separate assembly, check it for continuity with a meter. The usual arrangement is for the external overload to connect to the common terminal on the compressor. The relay connects to the start and run terminals. Thus the overload sees current through both or either winding and will open if the compressor draws too much current.

WITH THE REFRIGERATOR UNPLUGGED and the relay/overload assembly removed to expose the compressor terminals check the compressor for open or shorted windings. Normally the highest resistance reading will be between the S (start) and R (run) terminals since one is reading through both windings. The next highest reading is from S to C and the lowest reading should be from C to R. The resistance from S to C and from C to R should add to equal the reading from S to R. If C to R is open (no continuity) either the windings is open or the internal thermal overload is open. The thermal overload opens to prevent damage to the compressor from heat. If there is no continuity from C to R and the compressor is cold, the winding is open or the overload is defective and the compressor will have to be replaced. If S to C is open (no continuity) the start winding is defective and the compressor is bad. If the resistance from S to R is less than the sum of the resistance from S to C and C to R, the windings are shorted and the compressor is bad. If any terminal shows a low ( or zero) resistance to the case of the compressor, the windings are shorted and the compressor is bad.

Fig 1 Most Common pin arrangement on a compressor

The above resistance checks can verify that the windings are good and not shorted and that the electrical components inside the compressor are likely good, but it does not permit us to know if there is internal mechanical damage such as a seized bearing. The good news is that starting circuits fail much more frequently than compressor bearings so if the resistance readings of the compressor look normal, I would replace the starting circuit before condemning the refrigerator.

Keep in mind that even if the starting circuit and compressor are good the compressor will not have voltage to start if the cold-control or defrost timer have failed. I have been given more than one refrigerator with a bad compressor that had a defrost-timer stuck in the defrost position. That said, if you hear the typical humming of the compressor trying to start then cold-control and timer are sending voltage to the compressor start-circuit.