Page 204 - Boiler_Operators_Handbook,_Second_Edition

Refrigeration & AC 189

HEPA Filters on the exhaust air to protect the wheel) and they’re fre-
A final element added to some systems where the quently equipped with a number of pressure sensors to
air quality is critical are HEPA (high efficiency particu- detect problems of leakage and blockage.
late arresting) filters and they’re normally installed on
the outlet of the air handling unit. Since it’s on the pres- Complete Your Design Charts
sure side it ensures there will be no infiltration of air after The last line to draw on your psychrometric chart
the HEPA filter. The pressure drop required to achieve is from the equipment outlet condition to the room con-
the filtration would be added to the negative pressure in dition and that represents the heat and moisture added
the apparatus casing which would make in-leakage of to the air as it enters the room and mixes with the room
unconditioned air more of a problem along with requir- air to absorb the heat and moisture that’s generated in
ing a stronger casing. I know I mentioned the synthetic the space. Once you have charts for all the design con-
fiber factory and HEPA filters were used there to prevent ditions you can use copies of them to compare to actual
dust particles binding to the fiber during stretching and operation when problems arise. If you take actual mea-
spinning because that would cause the fiber to break. surements under full load conditions save those charts as
HEPA filters can actually be so fine that they can remove well because they’re “as-builts” not design situations and
odors. It’s another thing that requires attention to the a better reference to use. The problem is that we seldom
instruction manual. reach actual design conditions and have the time to take
the readings at exactly that moment. Don’t forget that you
Heat Exchangers, Heat and Energy Wheels have at least two design conditions, summer and winter.
Applications that require very high percentages of Now you have something like a steam cycle or re-
outside air are frequently equipped with some means frigerant cycle only it’s an air cycle. Label the chart with
of exchanging some energy between the outside air and the equipment name or number, copy it and put it away
exhaust air. Heat pipes were mentioned earlier. Dilution for the day when you’ll need it. Do a couple of others
of pollution of the conditioned space by manufacturing where conditions are different so you get a feel for what
processes and concern for cross contamination of work happens there and gradually build your library of the
are typical situations where 100% outside air is used. I’ll design and normal operating conditions for each piece
resist commenting on the former but I did have the op- of air conditioning equipment so you have the informa-
portunity to design air conditioning for a forensics lab- tion you need at hand and fully understand what’s sup-
oratory where contamination of samples was a concern posed to happen.
and they used 100% outside air. My design incorporat-
ed a simple heat exchanger that kept the two airstreams Air Handling Units
isolated and ensure no cross-contamination. The typical AHU consists of a mixing box where
Heat and Energy Wheels are what the name im- return and outside air are mixed before entering the fil-
plies. The heat wheels contain a fine lattice of metal that ters, a filter box, face and bypass dampers (if included),
absorbs heat from the hot air in summer and dumps it an access space for maintenance of the cooling coil, cool-
into the cooler exhaust air then absorbs heat from the ing coil, another access space for both cooling and heat-
exhaust air in winter and heats the cold outside air. En- ing coils, heating coil, humidifier, and fan housing. Most
ergy wheels, sometimes called enthalpy wheels, use a AHUs are fitted with a fan driven by a shaft connecting
desiccant coating that allows the wheel to absorb and to a sheave outside the unit with the motor, sheave, and
discharge moisture as well. The wheel is installed in belts. These are typically prefabricated in sections for
a casing with outside and exhaust air inlet and outlet field assembly but can also be shipped as an assembled
connections so as it rotates it picks up heat (and mois- unit that is simply rigged into place. Most AHUs use
ture) from one air stream and discharges it to the other. chilled water in the cooling coil and steam or hot wa-
I believe you can tell that this application doesn’t dif- ter in the heating coil. Valves controlling the flow of the
ferentiate between heat and moisture and other things heating and cooling mediums have to be added along
that can be in the air so it’s possible that contamination with diffusers, supply, return, and outside air ductwork
from the conditioned space can be returned to the out- to complete the installation. Openings for access to in-
side air drawn into the unit. A particular problem with spect and maintain the filters, fan and coils are provided
the wheel units is maintaining the seals that separate the in the casing. A drain pan is provided at the bottom of a
two air streams. Secondary problems include blockage cooling coil to collect and direct the condensate formed
due to filter leakage (yes, some of them require filters as moisture is removed from the air.