ACtion Magazine - July/August 2017 - 21

HD and OTR vehicle service: Keep it clean
consistently above its boiling point. Heat cycles
and diffusion help this process.
Some techs don't replace a dryer on an
empty system, others are still under the impression that moisture can be removed from a dryer
during evacuation. This is false. Once desiccant
absorbs moisture, it is beyond the capability of
your vacuum pump to remove it.
The reasoning for re-using a dryer is perplexing. Most of us warranty our work and
attempting to save the customer money by reusing a dryer is akin to sabotaging your repair
before it even goes back into service.
A dryer holds only a few drops of liquid
water when brand new. When fully hydrated,
operating conditions can cause the desiccant
to degrade. Extreme temperatures affect the
desiccant in the same way. High heat causes
the desiccant pellets to break apart into chunks,
then powder. This powder can circulate through
the system. Severe cold temperatures cause the
moisture to freeze, breaking the desiccant beads
apart further, and again into powder. The vibration level of HD and OTR vehicles helps this
process along.
Red Dot recommends HD and OTR sys-

tems have the dryer replaced annually. When
considering the vibration, harsh conditions and
pressures where these systems operate, this interval is reasonable.
*Rule of thumb: Desiccant in a dryer will
be saturated with approximately three years
use. Replace the dryer at least that often,
perhaps more often if the operating environment warrants.
Have you ever received a new part in the
box with the dryer and the caps rolling around
separately? If a new dryer arrives in that condition, return it to your supplier. The desiccant is
saturated from air it has been exposed to since
those caps fell out. ❆

Mike Bailey

Check your vacuum pump often using a
thermistor vacuum gauge. Attach it to the pump
and measure the ultimate vacuum your pump
can pull with fresh oil. Remember that number as the best vacuum level your pump can
achieve. A pump will typically pull higher micron numbers (less vacuum) as the oil becomes
contaminated with water.
*Rule of thumb: Change your pump oil
after ten hours of running time and more
often when working in high humidity conditions.
When choosing a vacuum pump to service
HD and OTR, consider that systems will vary
greatly in size. An oversized pump can be used
on a small system, but an undersized pump cannot be used on a large system. A shop servicing
HD and OTR would best use a 6 CFM pump. A
6 CFM pump can effectively evacuate a 50-ton
system which covers all potential systems to be
serviced.
Testing has shown that even when the lowside gauge reads 29 inches of vacuum (where
achievable), the inside of the system has not actually reached that level of vacuum. The gauge
reading more indicates the vacuum level in the
service hose, not in the system.
Consider that the small openings in the service valve cores are major restrictions. It is very
difficult to reduce the actual system to the reading on your gauge, and even with a very good
vacuum pump, it can take a long time to pull the
entire system into a true deep vacuum.
When it comes to removing moisture, the
entire system, not just the service gauge reading,
must be at a true, deep vacuum.
Thus, the bottom line is: Even the best vacuum pump or recovery/recycling machine may
not remove moisture through evacuation. The
best assurance for control of excess moisture in
the A/C system is to replace the desiccant.

Temperature chart in the RED zone
closest to the center?

You have enough on your mind,
so let us take the worry out of your air conditioning needs.

Dryers and accumulators
(Let's call them both dryers to keep things
flowing.)
Everyone has heard that the dryer or accumulator needs to be replaced WHENEVER
THE SYSTEM IS OPENED. I speak with
some technicians who still only replace the dryer or accumulator if the system came in empty.
Consider how the R-134a system is in a VACUUM whenever its temperature is below the
refrigerant boiling point of -15.3°F (-26.3°C).
Moisture enters with a small amount of air possibly past O-rings, compressor seals or loose
service caps, also entering when the system is

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July/August 2017 * ACTION

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Table of Contents for the Digital Edition of ACtion Magazine - July/August 2017