Fläkt Woods
3099 US 03.02
17
Specifications are subject to alteration without notice
PUMA (A–F) Rotary heat exchanger
TECHNICAL HANDBOOK
Project design advice
Location of the fan
Question: Is air recirculation permissible?
If air is recirculated, the fans can be located in any posi-
tion. If air recirculation is not permissible, the fans
should be installed as shown in Fig. 7 or Fig. 8 if particu-
larly high purging pressure is required.
Note that the installation shown in Fig. 7 may cause
negativ pressure in the building during the winter.
This is the most common location of the fans. The pres-
sure can be lowered by installing an adjusting damper in
the exhaust air duct upstream of the heat recovery unit.
Question: Is maximum cooling energy recovery desirable?
If the fans are installed as shown in Fig. 8, all of the losses
in the motor and the exhaust air fan and almost all of the
losses in the motor and the supply air fan will be dis-
charged with the exhaust air.
These locations give constant pressure conditions in the
building throughout the year.
Maximum cooling energy recovery will be achieved if the
fans are located so that the heat in the outdoor air and
the fan-generated heat are both transferred to the exhaust
air. This location is also suitable for premises in which
high air cleanliness is demanded.
p3
p1
p4
p2
Rum
p3
p1
p4
p2
Rum
Room
Room
Fig. 7
Fig. 8
Symbols used:
Rotor pressure drop on the supply air side,
∆
p1-2, inches.WG
Pressure differential across the purging sector,
p1 - p3, inches.WG
Supply air flow downstream of the heat exchanger,
q2, CFM
Exhaust air flow upstream of the heat exchanger,
q3, CFM
Exhaust fan air flow q4
Leakage flow, ql, CFM
Carry-over flow, qm, CFM
Rating factor for exhaust air =ƒ
Calculation example:
Given: PUMB-240 installed as shown in Fig. 5
q2 = q3 = 18 000 CFM
∆
p = 0.66 in.WG
p1 - p3 = 1.6 in.WG
From Chart 5, ql = 540 CFM
From the previous page, the carry-over air flow
would amount to approximately 2% of the supply air
flow, q2 (3% at 0.4 inches WG; 1.5% at 0.8 inches WG.
The exhaust air fan thus operates at a flow which is
around 5% higher than the exhaust air flow rate from the
room (q3).
q
4 =ƒ*
q
3
ƒ = 18000 + 540 + 0.02*18000 = 1.05
18000
q
4 = 1.05 * 18000 = 18900 CFM
q
3
+q
1
+q
m
ƒ=
q
3