source. Under these conditions, the chiller will be
operating at a higher head pressure than normally
encountered. At these high head conditions, the
centrifugal compressor may surge at part-load
conditions of as high as 30 percent to 40 percent
depending upon the conditions to which the chiller
is subjected. In these cases, the designer should
survey the manufacturers to determine at what load
the available chillers will surge, at the conditions
and loads to be encountered at the site. The
bracketed sentences will be removed from the
centrifugal chiller paragraph and replaced with the
appropriate capacity control requirements. The
designer should also consider multiple chillers to
satisfy the load and to partition the loading to the
chillers such that the heat recovery chiller load is
sufficiently high to avoid surge. When examining
heat recovery, full consideration should be given to
the effect of 35-40 degrees C (95-105 degree F)
water and the resulting power requirements of the
chiller on the economic benefit of heat recovery.
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Centrifugal compressors shall be single or multistage, having dynamically
balanced impellers, either direct or gear driven by the compressor driver.
Impellers shall be over-speed tested at 1.2 times the impeller-shaft speed.
Impeller shaft shall be heat-treated alloy steel with sufficient rigidity
for proper operation at any required operating speed. Centrifugal
compressors shall include:
a.
Shaft main bearings that are the rolling element type in
accordance with ABMA 9 or ABMA 11, journal type with bronze or
babbitt liners, or of the aluminum-alloy one-piece insert type.
Bearings shall be rated for an L(10) life of not less than 200,000
hours.
b.
Casing of cast iron, aluminum, or steel plate with split sections
gasketed and bolted or clamped together.
c.
Lubrication system of the forced-feed type that provides oil at
the proper pressure to all parts requiring lubrication.
d.
Provisions to ensure proper lubrication of bearings and shaft
seals prior to starting and upon stopping with or without electric
power supply. On units providing forced-feed lubrication prior to
starting, a differential oil pressure cutout interlocked with the
compressor starting equipment shall allow the compressor to
operate only when the required oil pressure is provided to the
bearings.
e.
Oil sump heaters controlled as recommended by the manufacturer.
f.
Temperature-or pressure-actuated prerotation vane or suction
damper to provide automatic capacity modulation from 100 percent
capacity to 10 percent capacity. If operation to 10 percent
capacity cannot be achieved without providing hot-gas bypass, then
the Contractor shall indicate in the equipment submittal the load
percent at which hot gas bypass is required.
SECTION 15620A
Page 29
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