Heavy Duty Mobile Air Conditioning Systems
Heavy-duty air conditioning applies to equipment on: subway
trains, buses, trucks, rail cars, airplanes, agricultural
equipment, underground mining equipment, large overhead cranes,
centrifugal chillers on ocean-going ships, and any large mobile
air conditioning equipment that does not fall into the categories
discussed in the previous sections.
6.1 Design
The equipment manufacturer should ensure that the design of
mobile air conditioning units incorporates a series of proven
features that will eliminate emissions to the atmosphere. The
same principles in Sections 2 to 5 apply here.
Compressors for some of the equipment using heavy-duty air
conditioning such as underground scoop trams and agricultural
equipment are essentially the same as large automotive
compressors with longer hose runs, and in some cases, external
condensing units. Some underground compressors are driven by
hydraulic motors. Trains and subways tend to use large commercial
semi-hermetic or belt-driven open compressors for air
conditioning. Buses on the other hand, use belt-driven
compressors with a mixture of copper and elastomeric hose
connections.
Compressors are subject to the same problems listed in
Sections 2, 3, 4, and 5.
Mechanical Seals (Open Drive) . The unique
environmental, geographic, and extreme hot and cold temperature
conditions
that mobile refrigeration equipment may be subjected to, can
be damaging to the mechanical seals of compressors for large
trucks, trains, agricultural equipment, etc.
Mechanical Shaft Seals. There are several factors that
can lead to the premature failure of mechanical seals. Exposure
of refrigerant and oil mixtures to various contaminants, the
presence of rust or particulate matter, and shaft misalignment
are some of the factors that cause seal failure and the resulted
leaks.
High Head Pressure . High head pressure is caused by
high outside ambient air temperatures, air in the system, and/or
condenser coils blocked with bugs, fluff, dirt, and debris.
Higher than normal operating pressure can cause leaks, emissions,
and premature equipment failure. In addition, the presence of air
and moisture can cause acid generation and oil breakdown that can
lead to premature equipment failure and refrigerant leakage.
Design features should include a method to alert the operator
of potential problems before they occur, so that corrective
action can be taken.
Refrigerant Moisture Tolerance. Virtually all of the
new replacement refrigerants and their respective oils have
little or no ability to tolerate moisture. Every precaution
should be taken to prevent moisture from entering the system.
Vibration . Vibration stress leaks should be minimized
by using:
- antivibration mountings;
- heavy-duty insulated clamps;
- metal braided vibration eliminators between fixed piping
and components that are subjected to movement; and
- a minimum number of soldered joints.
Protection of System. Design should provide for as much
physical protection as possible without compromising
accessibility .
6.2 Mobile Air
Conditioning Design Features
6.2.1 Design Features
Desirable design features should include:
- self-reseating pressure relief valves vented to the
outside of any enclosed space; and
- the use of adequate isolation valves and access fittings
to facilitate maintenance, repair, recovery, and
recycling of refrigerant; this will reduce service time
and allow the component to be replaced or serviced
without compromising the entire refrigerant charge.
6.2.2 Mobile Centrifugal Chillers
The chillers used on ocean-going ships are the same as those
on land with a few exceptions. The condensers used on ships are
the same as those found in commercial and industrial
applications. To prevent fouling and scaling of the primary
refrigeration condenser, the use of a secondary heat exchanger,
which uses sea water to absorb the heat being rejected and to
cool the refrigerant condenser cooling water down, should be
used.
Sacrificial anodes should be placed in the salt water heat
exchanger to help prevent corrosion, using the same technology as
the semihermetic and reciprocating refrigeration compressors
found aboard ships. Both the condenser and the heat exchanger
should be designed for easy maintenance and cleaning.
6.2.3 Excessive Water Velocity
Excessive water velocity through the tubes of the shell and
tube units can cause vibration or erosion failures and should be
avoided. Follow manufacturers’ recommendations and
guidelines.
6.2.4 Condenser Water Treatment
Condenser water conditions vary widely. Proper water treatment
and filtration will help minimize the effects of corrosion or
erosion that can cause failures and leaks.
6.3 Pipelines
and Connections
All pipelines should be designed so that the number of joints
are kept to a minimum. Flared fitting joints should not be used.
Welded, brazed, or flanged fittings are preferred. Use of
elastomeric hoses should be minimized to eliminate diffusional
leaking. Other options such as flexible metal tubing should be
considered.
6.4 Planned
Preventative Maintenance
Particulate matter and certain types of soils can damage the
air conditioning system by allowing moisture into the system
resulting in contamination of refrigerant and oils and leading to
leaks.
Planned preventative maintenance is the key to minimizing
breakdowns, down time, and refrigerant loss. The same information
found in the previous sections apply here.
Fluorescent dye leak detection methods have been proven
valuable particularly for underground scoop trams and rail coach
applications.
This equipment is subjected to the most adverse physical and
environmental conditions; therefore, it is imperative that units
be kept clean.
Refrigerant should be removed from rail coaches and other
equipment that are out of service for extended periods of time. A
dry nitrogen charge should be used to keep a positive pressure in
the system. The pressure is monitored during the shutdown period.
When the system is reactivated, it should be evacuated and the
oil changed. A vacuum pulled down to less than 500 mµ of Hg is
recommended before the system is recharged.
6.5 Systems
Conversion to Alternate Refrigerants
Follow the principles and guidelines detailed in Sections 2.8
and 3.6, and the subsequent sections.
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