AAON LZ Series, Installation Operation & Maintenance

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69
Water Quality
Table 15 - Recirculating Water Quality
Guidelines
PH 6.5 to 9.0
Hardness as CaCO
3
500 PPM Max
Alkalinity as CaCO
3
500 PPM Max
Total Dissolved
Solids
2000 PPM Max
Chlorides as NaCl 1500 PPM Max
Sulfates 750 PPM Max
Cycles of concentration (the ratio of
dissolved solids in recirculated water to
dissolved solids in makeup water), should be
determined and monitored frequently by a
competent water treatment expert.
To limit cycles of concentration to maintain
the above guideline, it is necessary to “bleed”
a certain portion of the recirculated water.
This is achieved automatically with a
solenoid valve actuated by a conductivity
meter set at the desired conductivity
corresponding to the desired cycles of
concentration. It should be noted that these
are guidelines and even though these
individual values are met, under certain
conditions the water quality can be
aggressive. For example, water with very low
alkalinity and levels of chlorides and sulfates
approaching maximum recommended levels
can be corrosive.
All AAON evaporative-condensed chillers
are furnished with a bleed system fitting and
valve to continuously remove a small portion
of the recirculated water to keep the water
quality within the above listed parameters.
This device is located on the discharge side
of the pump. It is important to note that since
“bleed” rate is a function of evaporation rate
(i.e., amount of heat rejected), if the bleed
setting is manual based on design heat load,
too much water will be removed when the
heat load is less that design.
The AAON evaporative-condensed chiller is
equipped with a de-superheater. The de-
superheater coil is located above the mist
eliminators. Approximately 22% of the total
heat of rejection is accomplished with the de-
superheater. Water usage of the AAON
evaporative-condensed
chiller is
approximately 22% less than evaporative-
condensed chillers not equipped with a de-
superheater.
One method of calculating evaporation and
bleed in gallons per minute (gpm) is shown
as follows:
Evaporation Rate
=
Total Heat of Rejection via Evaporation
h
fg
btu lb ⁄ ×ρ lb gal ⁄ ×60 min hr ⁄
=
Total Heat of Rejection via Evaporation
525,000 (Btu hr
⁄
gpm)
⁄
Bleed Rate
=
Evaporation Rate
Cycles of Concentration − 1
Example:
A unit has 120 ton cooling capacity with a
compressor EER = 15
Total Heat of Rejection
= Unit Capacity in Tons × 12000 ×
(1 +
3.413
EER
)
= 120 × 12000 ×
(1 +
3.413
15
)
= 1,767,648 Btu/hr
Total Full Load Heat of Rejection via
Evaporation
= Total Heat of Rejection ×
(1 –
fraction of heat rejected by
desuperheater)
= 1,767,648 Btu/hr × (1 – 0.22)
= 1,378,765 Btu/hr