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Sealed Sytem Factors

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System Characteristics
Chapter 5 Page 1

Before Entering a Sealed System: Once all external causes for the reduction in cooling capacity are eliminated, a closer look at the system is in order. Follow these steps to verify a system problem: • Remove the back cover to the machine compartment and feel the compressor discharge line. - A very hot line indicates that the compressor is operating under extreme duress (high ambient, overcharged, very high heat load). - A cool line indicates that the compressor is not doing much work (leak, very low ambient, evaporator plugged with ice). • Check compressor current draw. Current draw should match the conditions that you encountered in first two steps. - Low current draw could indicate a loss of refrigerant, an inefficient compressor, a restriction (cap tube plugged), or lack of air movement through the evaporator - High current draw could point to high ambient, high heat load (such as a light on in the refrigerator), non- condensables in the system, defective compressor, lack of air movement through condenser or an overcharge. • Feel the top of the compressor (compressor dome). - If too hot to touch but there is no cooling occurring, suspect air (non-condensable) has entered into the system. - If cool to the touch, compressor is not doing very much work and the system may be out of refrigerant or have evaporator air flow problems. • If these further checks indicate that the problem is indeed with the sealed system, tap into the system.

Refrigerant Charge - The system has to be properly charged with the factory recommended refrigerant. If the system looses any of its refrigerant, cooling capacity will be affected. A system leak can take several years for enough refrigerant to escape from the system to affect the performance of the unit. A modern refrigerator is typically charged with less than 6 oz of refrigerant. That’s not a whole lot, especially compared to the older R-12 units that could hold as much as 20 or 22 ounces of refrigerant. But even with these smaller charges, if the system only loses ½ oz per year, it could take 2 or three years before the customer notices a problem. The chart below shows what happens to cooling capacity of a 950 BTU compressor when a system charge drops below 75% of nominal. Notice that at 75% of correct charge, the cooling capacity (dashed line) is only about 600 BTUs. The system is undercharged by 25% but the BTU rating drops by 36%. Leaks normally occur at the joints where the


system tubing is brazed together. If leak occurs on the suction side of the compressor, this is referred to as a low side leak. If the leak occurs on the discharge side of system, this is referred to as a high side leak. Low side leaks are especially harmful to a system because once the low side drops into a vacuum, the compressor begins to pull air from the kitchen into the system (through the hole in the tubing). Air is detrimental for a couple of reasons:

Air is a non-condensable. The air travels through the system and once in the condenser, no amount of pressure or cooling will force the air to condense into a liquid (thus the non-condensable label). Pressure in the condenser rises significantly and cooling stops. The air that enters the system contains moisture. As we will learn later in this section, moisture creates its own set of problems for a sealed system. If the system has been previously repaired and incorrectly charged, an overcharge can severely affect the operation of a refrigeration system. Just as an undercharge reduces the system’s ability to absorb heat, an overcharge has a similar impact on cooling efficiency. Moreover, overcharges have the added impact of greatly increasing power consumption. The chart above clearly shows that missing the correct charge by 5% in either direction (over or under) reduces cooling capacity of the compressor to about 900 BTU. But notice what happens when the system is overcharged. Not only is capacity diminished but power consumption rises by 20%. At a 10% overcharge, the power consumption rises 44%. Unfortunately, on a system that only holds 5.5 oz of refrigerant, it only takes an extra ½ oz of refrigerant to overcharge the system by 10%. Considering that the hoses of a manifold gauge set can hold up to 2 oz of liquid refrigerant, overcharging a system by a ½ oz is not difficult. Compounding the problem is that many technicians feel that if 5.5 ounces in the system is good, 6 ounces will be even better. In this case, more is not better. The added refrigerant raises system pressures and affects the boiling and condensing points of the refrigerant. Moreover, the compressor is forced to work harder because it has to pump against higher pressures. The rule is, if the system charge calls for 5.5 oz of refrigerant, there should be 5.5 oz in the system. No more, no less! But, as we’ve learned from the above chart, if we miss the correct charge at all, it’s better to be slightly undercharged than to be overcharged by any amount. Contaminants and Moisture: The system must be clean of contaminants and free of moisture. Moisture in a system is especially harmful. Three major issues arise with moisture contents that exceed 300 microns of pressure: Water mixes with the refrigerant to form hydrochloric and hydrofluoric acids. These eat away at the insulation on the compressor windings and eventually shorts out the motor. It’s not a question of if the compressor will fail; it’s a matter of when. It may take several months or even a couple of years but the failure is inevitable. Water mixes with the refrigerant to form salts and sludge that could plug the capillary tube. If refrigerant doesn’t flow through the cap tube, cooling stops. If there’s enough water left in the system, the water could create a condition called a floating restriction. This type of restriction occurs when moisture travels with the refrigerant into the evaporator. There, the sub zero temperatures in the freezer cause the water to freeze. Ice forms on the end of the cap tube, blocks the flow of refrigerant and cooling stops. Once the freezer temperatures begin to rise, the ice melts off the end of the cap tube and the flow of refrigerant resumes. The restriction “floats” through the system and plugs the cap tube intermittently. The customer complains of food loss and erratic temperatures.

Normal Operating Cycle: Before we examine common sealed system failures, let’s briefly review the normal operating conditions of a R-134a system. The conditions depicted above can be used as a basis of comparison for deviations from the norm caused by a failure. Specifications of a typical R-134a refrigerator: Freezer compartment temperature- Normal range is 0º F,+/- 3º F Fresh food compartment temperature- Normal range is 38 º F, +/- 3 º Compressor run current- Depending on BTU size of compressor and how hard it’s working, current draw will range from about .5 amps to 1.5 amps. Low side pressure- 0 PSIG. Actual pressures will range from 10 inches of vacuum at startup to about 7-8 PSIG in high ambient conditions or with an extreme heat load. High Side pressure- 125 PSIG- Again, depending on conditions, actual pressures will vary and may range from 100 PSIG under very low load conditions to 150 PSIG in high ambient or high load conditions. Frost pattern- Full (after the unit has been running) Defrost - Tube and fin evaporators must be defrosted on a regular basis to prevent ice from plugging the coil. (Frost on the coil is normal. Ice buildup on the coil indicates that a defrost failure has occurred.) Condenser liquid level- Varies but last 2-3 passes of condenser should feel cooler to the touch – if heated the tubing should remain the same temperature. The liquid refrigerant will absorb the heat as it changes state. Condenser temperature- Warm at the inlet, cooler at the outlet Compressor temperature- The dome (top) of the compressor will range from warm to very warm to the touch. Compressor discharge line temperature- Warm to the touch (the greater the heat load and ambient, the warmer the line should feel) Suction line temperature- Ambient, plus or minus a few degrees

Normal System Pressures
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