Support
and the customer must feel that he is doing business with the
right company and the right person.
It is very important to discuss the issue with the customer and
determine:
• What is the nature of the fault? Is it permanent or
intermittent?
• Has any work been done on the plant recently or has anyone
had access to it?
• Are the operating instructions and wiring diagrams
available?
• Is a logbook kept for plant maintenance and repairs?
• Have there been any abnormal conditions such as power or
water outages?
• Anything unusual (burnt smell, noises or smoke)?
• From the information that the customer has given to you,
it may be possible to pinpoint the issue coupled with the
knowledge that you have.
For example:
• If there has been a power failure the plant may not have re-
started automatically.
• If someone else has worked on the plant, a fault may have
re-occurred or not been properly attended to.
• The customer may say the air-conditioned area is ‘stuffy’ –
this relates to air flow.
• If there is a burnt smell, slipping V belts may be the cause.
• If there has been a water failure and the refrigeration plant
is water-cooled it may have tripped on high pressure or flow
failure.
• Noises may point to bearings, loose fans, couplings.
• The fault will usually be of a mechanical, electrical or control
nature so let us look at these aspects individually.
There are many fault-finding
guides available.
MECHANICAL ASPECTS
Before proceeding with mechanical fault finding ensure that
you have complied with all safety requirements. You should
have observed any obvious mechanical fault in your initial
investigation, such as V-belt failure or blocked filters, broken fans
and so on.
Possible causes of air flow failure could be:
• Check that fan belts are not worn and slipping or that pulleys
/ fans are not loose on the shafts.
• Check for closed fire dampers or incorrectly operating
mixing dampers.
www.hvacronline.co.za
•
•
•
Check for unit panels or access doors left open causing air to
bypass the cooling coil.
Foreign materials blocking airflow in ducts, coils, heaters,
louvres, inlet screens.
Reduced airflow caused by incorrect rotation of fans, loose
drives, dirty filters or other factors may cause the cooling coil
to ice up.
If the airflow through the system is correct, one may move on to
the refrigeration equipment within the plant.
With regards to the refrigeration cycle you may encounter:
• The refrigeration plant has lost some or all of its refrigerant
charge and tripped on low suction pressure. This is a very,
very common problem.
• Do not add refrigerant until you have found and repaired the
leak.
• Consider the consequences of the leak with regards to
refrigerant blends.
You may find that the refrigeration plant has tripped on high
discharge pressure due to a condenser fault. If the condenser is
air-cooled, you may find an issue with the fan. You may also find
that the condenser coil is blocked by dirt or an obstruction for
example: newspaper is blocking the airflow.
Air (non-condensable) may have entered the system resulting
in high head pressures. If the condenser is water-cooled is there
sufficient water? If so, then further causes could be a faulty
condenser water pump, blocked water strainer, failed check valve,
closed valve on the condenser water circuit or faulty cooling tower
fan. Possibly a cooling tower water bypass valve has stuck thus
causing the condensing water to bypass the cooling tower in-turn
causing high head pressures.
Water-cooled condensers can accumulate scale build-up
on the heat exchange surfaces. Typically, low condenser water
temperatures in and out of the condenser together with high
discharge temperature of the refrigerant can be observed. This
relates to a service issue. Problems relating to chilled water flow
may be the cause of plant failure.
As with water cooled condenser systems, problems may be
experienced with a faulty chilled water pump, blocked water
strainer or a failed check valve.
A chilled water system is usually filled through an open tank
above the highest point in the chilled water circulation piping
and equipment. This tank (referred to as the ‘make up tank’) also
serves as an expansion tank to accommodate system expansion
when the water content heats up. Ensure that the mains water
feed ball valve is functioning to top up water content. If the
tank does not remain full one can experience air-locking, pump
cavitation and loss of circulation may occur, this will cause the
entire plant to trip on ‘No Flow’.
RACA Journal I May 2020
59