These are not nearly so common nowadays. They should however be fitted at the final discharge to atmosphere where this is vertically up. The ducting must be protected from the ingress of rain. In former times, they were known as “Chinamen’s hats” — a descriptive term with no racial connotations!
The smaller the diameter of this cap, the lower it must be fitted to the duct end to prevent rain ingress. But the lower it is fixed, the greater its resistance. The resistance is also affected by the design of the inverted cone.
Two accepted designs have been tested as shown in Figure 3.70. Design B is American and rather high in the gap. For British weather conditions it should probably be fixed slightly lower, when the pressure loss should not exceed 0.25 x pv in the duct.
The static pressure loss for Design A is 1.0 x pv
The static pressure loss for Design B is 0.2 x pv
If the velocity is high in the discharge duct to atmosphere, as in dust collecting systems, a tapered diffuser should be fitted before the weather cap.
As an example:
Consider a straight duct discharging at 20 m/s i. e. p„ = 240 Pa. The loss in a cap to Design A would then be 1.0 x pv = 240 Pa. Now assume a tapered duct is fitted with an included angle of say 7° and an area ratio of 1.75 to 1, which is a reasonable design.
From Figure 3.20 (in Section 3.3.2 on diffusers) with discharge direct to atmosphere and interpolating to 7°, the static regain is about 0.5 x pv = 120 Pa. Resistance of the weather cap is reduced due to the lower velocity, which at 1.75 x area of duct would be about 11.4 m/s with a velocity pressure of 78 Pa, which would probably offset the frictional resistance of the length of tapered duct.
In general it may be taken that by the use of a taper with a larger weather cap, the discharge resistance in such cases can be eliminated with consequent saving in absorbed power
Posted in Fans Ventilation A Practical Guide