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Design & installation Guide For McQuay MDS Multi System
3.4.2 Calculation of the air pipe resistance
The air resistance in the air outlet and return air inlet cause pressure loss when the air pipe is
being used, and the unit can supply air only when it overcomes the resistance. It is equal to the air
pressure of the unit with pressure loss, that is, exterior static pressure (ESP).
The pipe resistance must be equal to the ESP. Otherwise, or the air volume will be too large or
small causing the unit faulty.
To determine the ESP of the unit, get the whole-static pressure of the air pipe. In this case, it is
generally required to calculate the static pressure loss at the farthest point of the air pipe or the
maximum static pressure.
■ Calculation method of the air pipe resistance
● Preliminary determination of the air pipe resistance:
Air pipe resistance=R
m
L(1+K) (Pa)
R
m
=0.8~1.5Pa/m
L: Total length of the air pipe with the loop of most resistance, m
K: Ratio of local resistance loss to frictional resistance. If there are few local components,
k=1~2; if there are many local components, k=3~5
● Draw the ventilation system diagram, mark the air flow by segment and number each pipe
segment.
● Assume the air speed (refer to annex 4), and calculate the cross-section dimension and the
pipe diameter.
The conflux of the return air inlet brings little affect on the air flow pattern in the room. So, its structure
is simple. Some only have a metal mesh in the orifice, or a shutter or grille.
● The return air inlet location and shape depend on the air flow pattern. If it is installed in the lower area
of the room, the lower edge of the inlet must be at least 0.15 m away from the ground.
● The return air inlet structure may be simple, but should be equipped with air volume adjustment device.
■ Size of the air outlet
Determine the air volume and type of a single air outlet according to the number of air outlets to be
installed and the volume requirements in the room where the air conditioner will be installed. Make sure
there is no overlapping between the set air volume and the minimum diffusion radius, and the maximum
diffusion radius covers the whole room. In addition, check whether the vertical arrival distance is
adequate, and determine the air speed in the air outlet at the acceptable noise level.
Air flow pattern diagram
Air flow pattern diagram
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