An air conditioning repair is *not complete until fresh *desiccant is installed, service experts said. Therefore, a new receiver-drier or accumulator should be included in the job estimate instead of being an afterthought to it. Desiccant is a moisture-absorbent material contained in the air conditioner's receiver-drier or accumulator.
Refrigerant, like brake fluid, readily absorbs moisture from the surrounding air. Because no refrigeration system is perfectly airtight, a ``good'' air conditioner still absorbs moisture over a period of time.
Depending upon the application, about 50 droplets of water will saturate the desiccant inside a receiver-drier or accumulator. But as little as one or two droplets can damage the refrigeration system.
Moisture threatens the refrigeration system several ways. First, water mixed with refrigerant forms highly corrosive acids that eat holes in metal air conditioner parts.
Second, mixing water with refrigerant oil creates sludge that may restrict or totally clog passages inside the system. Third, excessive moisture may pulverize desiccant into a fine powder that becomes a system contaminant itself.
Elsewhere in this service section, we explained that excessive moisture can freeze inside an expansion valve (TXV), blocking refrigerant flow. The freeze-and-thaw cycle excessive moisture causes in a TXV-equipped system creates an intermittent or erratic cooling complaint.
When moisture freezes on a TXV, frost forms around the TXV inlet. If you remember to look at your gauges before the frost thaws, you'll see the same reading you get when the TXV is stuck shut or completely clogged with debris: Low-side pressure drops into a vacuum (negative pressure).
Excessive moisture may also cause abnormally low pressure readings on both sides of the system. The pressure readings may only drop when you open the windows, put the blower on high and rev the engine.
High and dry
Many technicians forego desiccant replacement because they think evacuating the refrigeration system removes all moisture. Evacuating the system with a vacuum pump is supposed to lower system pressure enough to boil off moisture lingering inside the system.
Among other experts, Ward Atkinson disagrees with this premise. A renowned designer of original equipment air conditioners, Mr. Atkinson now does research and development work at Sun Test Engineering in Scottsdale, Ariz. He's also a technical adviser to the Mobile Air Conditioning Society (MACS).
He explained that the most powerful refrigeration vacuum pumps available cannot lower system pressure enough to boil off all the moisture. And although evacuation is an important service procedure that removes some moisture, it won't dry out old desiccant. That's why a new receiver-drier or accumulator is essential for protecting the system and preventing customer comebacks, he said.
When installing a new receiver-
drier or accumulator, keep its openings capped until you install the part. This reduces the amount of moisture the desiccant absorbs prior to installation.
If a compressor has failed or contamination is evident inside the refrigeration system, always flush the system per the manufacturer's recommendations with an approved flushing technique.
Many driveability technicians routinely cut open every fuel filter they replace to check for signs of fuel system contamination or deterioration. If they see evidence of fuel system damage, they alert the customer and indicate their findings on the work order to prevent unpleasant surprises later.
Likewise, many air conditioning specialists recommend an autopsy on every receiver-drier or accumulator. The neatest and quickest way to open up these parts is with a large exhaust tubing cutter. A hacksaw works fine, but it creates metal chips that may complicate the autopsy.
A heavy buildup of black rubbery material indicates flaking and deteriorating of the refrigerant hoses. Unless you replace the hoses, the
rubber debris will clog the system.
Metal particles usually come from a worn compressor and may point to a compressor failure waiting to happen. Of course, the sludge and scale resulting from excessive moisture also may collect inside a receiver-drier or accumulator.
Remember that if debris causes a substantial restriction inside a receiver-drier, it also will cause a temperature drop across it. Replace the receiver-drier if its outlet is noticeably cooler than its inlet.
Finally, the bulk of the debris in an accumulator-type refrigeration system will clog the orifice tube screen before it collects inside the accumulator.