Handbook of air conditioning and refrigeration / Shan K

30.20 CHAPTER THIRTY
temperature is still at 46°F (7.8°C) or above, the discharge air temperature sensor T2 detects the fall
in T dis and the DDC controller modulates the two-way valve of the chilled water cooling coil in the
MAU to reduce its water flow until a 47.5°F (8.6°C) discharge air temperature and a discharge air
dew point of 46°F (7.8°C) are maintained.
If the space sensible cooling load falls below the design load, the space temperature drops accordingly.
As the space temperature sensor T1 detects such a fall in temperature, the DDC controller
modulates the two-way valve of the sensible cooling coil in the RAU, reduces its water flow,
and tends to maintain a constant space temperature of T r � 69°F (20.6°C).
In addition to the MAU discharge air temperature and space temperature controls, there are controls
for space humidity, space pressurization, filter pressure drop monitoring, and smoke detection.
Because the discharge air dew point from the MAU is 46°F (7.8°C), the space dew point is also
46°F (7.8°C), and the mixture of recirculating and makeup air still has a dew point of 46°F (7.8°C).
After sensible cooling in the RAU, the dew point of supply air, point s, remains at 46°F (7.8°C). Because
the sensible heat ratio of the space conditioning line SHR s � 0.99, the space relative humidity
will be always around 42.5 percent if the space temperature T r is maintained at 69°F (20.6°C).
For a space like a clean room having only a negligible latent load, supply air dew-point control
can always maintain both space temperature and relative humidity within required limits by means
of sensible cooling and reheating when the space sensible cooling load is varied.
In Fig. 30.1a, there is a pressure sensor P1 in the clean room. This sensor measures the pressure
differential �p r between the clean room and the surrounding area. This pressure differential is generally
maintained at 0.05 in. WC (12.5 Pa) to prevent infiltration of contaminated air.
According to the signal from P1, the DDC controller modulates the opening of the interlocked
makeup air and recirculating dampers. When there is an increase in makeup air and a decrease in
recirculating air in the mixing box of RAU, the space pressure differential tends to rise. A decrease
in makeup air and an increase in recirculating air cause a drop in space pressure differential. The
supply of makeup air is balanced by the process exhaust air and the exfiltrated air from door gaps
due to the pressure differential.
Pressure sensors are also used for MERV 8 prefilters, HEPA filters, and ULPA filters. If the
pressure drop exceeds a predetermined limit, an indicating lamp flashes or an alarm is energized.
Both signals call for replacement.
When the smoke detector is energized, a signal is sent to the building fire control system. The
fire alarm and the smoke control system are energized accordingly.
Winter Mode Operation and Controls
When the dew point of the outdoor air T o� is 44 °F (6.7°C) or below, the clean-room system is in
winter mode operation. If T o��44°F (6.7°C) and T o � 53.5°F (11.9°C), the outdoor air is preheated
at the preheating coil to a temperature T ph � 53.5°F (11.9°C). After absorbing the fan heat
of the MAU, it enters the RAU at a discharge temperature T dis � 55°F (12.8°C). If T o��44°F
(6.7°C) and T o � 53.5°F (11.9°C), the outdoor air is sensibly cooled to 53.5°F (11.9°C). Again, it is
discharged to the RAU at T dis � 55°F (12.8°C) after absorbing fan heat.
If makeup air at a temperature of 55°F (12.8°C) and a dew point of 44°F (6.7°C) is mixed with
winter mode recirculating air of temperature 69°F (20.6°C) and 40 percent relative humidity (a dew
point of 44°F, or 6.7°C), the resulting mixture will be at a temperature of 67.5°F (19.7°C). The dew
point will still be 44°F (6.7°C). If the mixture is cooled at the sensible cooling coil to 62.5°F
(16.9°C) according to the signal of the temperature sensor T1, and if it is supplied to the clean room
at a temperature of 63.5°F (17.5°C) and a dew point of 44°F (6.7°C) after absorbing the fan heat of
RAU, then it will maintain a space temperature T r � 69°F (20.6°C), � r � 40 percent, and a dew
point of 44°F (6.7°C) at a maximum space sensible cooling load of 563,000 Btu/h (164,960 W). A
lower space sensible cooling load results in a decrease in the space temperature T r. As the sensible
cooling capacity at the cooling coil is modulated and reduced by the DDC controller, the space temperature
will still be maintained at 69°F (20.6°C).
If T o��44°F (6.7°C), space relative humidity � r may be less than 40 percent. When the space