Fläkt Woods
3099 US 03.02
9
Specifications are subject to alteration without notice
PUMA (A–F) Rotary heat exchanger
TECHNICAL HANDBOOK
The Process in the psychrometric chart
Summer operation
Charts 1 and 2 show summer conditions in which the out-
door air is warmer and more humid than the exhaust air.
The hygroscopic rotor (Chart 2) lowers both the moisture
content and the temperature to the vicinity of the exhaust
air conditions, and gives an enthalpy efficiency of 75%. The
nonhygroscopic exchanger (Chart 1) lowers the temperature
by the same amount, but does not change the moisture con-
tent. In this case the supply air enthalpy efficiency will be
only about 25%. The example illustrates the significance of
the high moisture efficiency of the hygroscopic rotor, above
all in humid, warm climates.
Winter operation
Charts 1 and 2 show a winter case with moderately low
outdoor temperatures. No condensation takes place in the
nonhygroscopic rotor, (Chart 1) which therefore does not
contribute to the moisture content of the supply air. On
the other hand, the hygroscopic rotor (Chart 2) raises the
moisture content of the supply air by almost 11 Gr/lb of
air, which usually offers welcome humidification of the
supply air. The nonhygroscopic rotor can operate without
risk of freezing even when condensation takes place at
temperatures below 32°F.
Frosting - Defrosting
Rotor temperatures below 32°F need not necessarily cause
frosting in the rotor. Moisture transfer then takes place by
the moisture, which has been deposited as frost on the
rotor surface, being evaporated on the supply air side. For
frosting to occur, there must also be excess water in the
rotor. This will take place if the supply air is not capable
of absorbing the moisture that has condensed out of the
exhaust air.
The frosting process, which causes an increase in pressure
drop across the rotor, normally takes many hours. The
frosting problem is therefore often relieved by the out-
door temperature varying over a 24 hour period, or
because the heat exchanger is in operation during only
part of the 24-hour period.
Frosting limit
Frosting will occur if excess water should occur, at the
same time as the supply air inlet temperature is below
14ºF. This temperature applies with relatively good accu-
racy at different airflow rates, full speed and typical
exhaust air temperatures occurring in comfort ventilation
systems.
Excess water will occur in the hygroscopic rotor as soon
as the interconnecting line between the inlet conditions
for the two air streams intersects the saturation line in the
psychometric chart (see Chart 3).
In the case of a nonhygroscopic rotor, excess water will
form when the interconnecting line between the supply
air condition and the exhaust air dewpoint plus approxi-
mately 7ºF, as shown in Chart 4, intersects the saturation
line in the psychometric chart.
Frosting time
As an example, it will take about 8 hours for the pressure
drop to increase by 50% if the saturation curve is inter-
sected as shown in Chart 3, and about 4 hours if the sat-
uration curve is intersected as shown in Chart 4.
Note that the frosting time will be as above if the temper-
ature and moisture conditions are constant throughout
the frosting time. But since the temperature often varies,
the frosting time may be appreciably longer. As a result of
factors such as operating time and supply air temperature
variations, experience shows that a minor intersection of
the saturation curve is permissible without significant
frosting occurring, even if the design outdoor tempera-
ture is below 14°F.
Defrosting - avoidance of frosting
Frosting can be totally avoided by preheating the outdoor
air to a temperature so that the line connecting indoor
and outdoor conditions in the psychometric chart falls
below the saturation line. Heating to 14ºF is normally
adequate. The rotor can be defrosted, normally within
5–10 minutes, in several ways.
– By reducing the rotor speed to around 0.5 r/min (see
example 5 page 22).
– By preheating the incoming outdoor air to around
23°F.
– By bypassing a sufficient amount of supply air across
the rotor so that the outlet temperature on the
exhaust air side will be at least around 41°F. As an
example, the supply air flow rate would have to be
reduced to around half for defrosting to take place at
the normal exhaust air temperature, at a 75%
temperature efficiency and an outdoor temperature
of about –4°F.
All three methods can be used for a variable speed rotor
drive, while the last two can be used with constant speed
drive.around half for defrosting to take place at the nor-
mal exhaust air temperature, around 75% temperature
efficiency and an outdoor temperature of about –4°F.
All three methods can be used for a variable-speed rotor,
while the last two can be used at constant speed.