Retrotec USACE - Manual (Page 201)

Appendix D D71
These conditions create tremendous ineffi ciencies:
The lack of resets for chilled water system differential pressure setpoint, ■
the chilled water supply temperature setpoint, and the condenser water
supply setpoint result in operating in ineffi cient modes.
The lack of standard operator interface and training results in an inabil-
■
ity to tune the systems properly, so setpoints get locked in at a value
that is known to work—very cold chilled water supply temperatures
and very high differential pressures.
By starting an additional chiller prematurely, the combined chillers
■
may be operating at 30–40% load each, while their constant-speed pri-
mary and condenser water pumps are all operating at 100% capacity
and energy consumption rate. Chillers operating at light loads with fi xed
chilled water supply and condenser water supply setpoints are extremely
ineffi cient. For further details, see Appendix J.
Note: If all of the chiller plant equipment that is operational is equipped with
variable frequency drives (VFDs), it is sometimes desirable to operate more
equipment when the refrigerant lift conditions are low but the loads are rela-
tively high. The added heat transfer surface area associated with starting an-
other variable-speed chiller can improve the system effi ciency under these
conditions.
D.4.2.14 Ineffi cient Dehumidifi cation/Reheat (Ineffi ciency)
Undersized cooling coils require extremely cold chilled water supply tempera-
tures that cannot be generated and delivered to the buildings at many sites.
Chiller plants sometimes generate very cold water (sometimes as low as 3.3 °C
(38 °F)) in an attempt to provide adequate cooling and dehumidifi cation.
However, by the time the water reaches the furthest load, it warms up
(sometimes as high as 8.9 °C (48 °F)). Additionally, cooling coil performance
degrades with time, so a coil that performs adequately may fail to perform as
the years wear on.
Supplying nearly saturated 12.8 °C (55 °F) air into a humid space results
in condensation in the space, which can lead to mold and mildew problems.
Electric or hot water reheat systems are sometimes employed to decrease the
relative humidity of the supply air to the spaces to reduce the potential for
condensation, but these systems are typically defeated in the fi eld to reduce
energy use.
Ineffi cient air dehumidifi cation results in signifi cant repair and mainte-
nance costs. The cost of mold remediation outweighs the total fi rst cost of the
HVAC system by an order of magnitude over the life of the HVAC system.
Changing dehumidifi cation/reheat system design strategy can reduce
energy consumption and cooling and reheat loads, improve IAQ, and reduce
the potential for mold growth in occupied spaces. These factors will reduce
the level, frequency, and expense of remediation that seems to occur on a
regular basis.