Page 202 - HVAC Pump Handbook

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Pump Drivers and Variable-Speed Drives

Pump Drivers and Variable-Speed Drives 199

where 3412 thermal equivalent of a kW in btu/h
E efficiency of the drive as a fraction at maximum speed

If the energy consumption of the drive is 100 kW and the efficiency
is 95 percent, the heat expended would be 100 3412 (1 0.95), or
17,060 Btu/h.
2. Cleanliness. Most commercial variable-frequency drives operate
very well in normal HVAC equipment rooms. Industrial applications
that are dusty may require special enclosures to keep the dust away
from the internals of the variable-speed drive. NEMA 12 enclosures
with internal air-conditioners may be required. The air-conditioners
are usually air-cooled, so provisions must be made to keep clean the
condenser coil on any air-conditioner.
Some terribly dusty or dirty applications should be equipped with a
water-cooled drive instead of a variable-frequency drive. The water-
cooled drive could be a slip-type drive using an eddy-current or fluid
coupling.
Sometimes a variable-frequency drive can be cooled with clean out-
side air through ducts that eliminate the dirt or dust problem.
Adequate duct size or auxiliary fans must be furnished so that there
is no external static on the ventilating fans inside the drive.
3. Chemical attack. Variable-frequency drives contain many copper
or copper-alloy parts that are susceptible to attack by acids or other
sulfur-bearing compounds. Again, most HVAC installations do not
have such atmospheres, but some industrial cooling and heating sys-
tems do. For example, very small quantities of hydrogen sulfide in
the air can have a deleterious effect on variable-speed drives. When
these conditions exist, special construction must be used such as
NEMA 12 enclosures with internal air-conditioners or ducted fresh air
as described above for dusty atmospheres (see Fig. 7.16a). The con-
denser coils for these air-conditioners must be coated with a compound,
such as heresite, that resists attack from the hydrogen sulfide.
4. Maximum temperature. The maximum ambient temperature for
most variable-speed drives is 104°F (40°C); for higher ambient tem-
peratures, internal air-conditioners or ducted cool air should be con-
sidered for these applications.
5. Location. Variable-speed drives should be located in dry areas
where they cannot be wetted by either surface water or water from
overhead pipes. When there is no other location than under water or
steam pipes, the cabinets should be equipped with drip shields. Never
locate variable-speed drives outdoors in the open where they are exposed
to sunlight! If it is necessary to install them outdoors, locate them
under a sun shield that will protect them from direct sunlight.

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