# Diffusers

These are attached to the discharge or fan outlet and are used to improve the fan static pressure of medium to high pressure fans. They may also be used in a system at the end of the outlet duct to atmosphere. A change from high to low velocity is accompanied by a conversion from velocity pressure to static pressure. There is always a loss in this conversion such that the total pressure is never the same before and after the diffuser. Efficiency of conversion E is never 100%.

E = percentage converted of difference in initial and final velocity pressures.

True efficiency of conversion in the diffuser itself depends almost entirely on the angle of taper. If however, the diffusing taper is followed by a length of straight ducting 4 to 6 diameters long, then there is some additional conversion after the taper. In such cases, the overall efficiency, as determined by test, is related to the angle of the taper and the area ratio.

The included angle of a jet of air which is confined by the walls of a duct is about 7°. If the taper is more than this then the flow leaves the walls and dead areas result.

An unconfined jet of air in free space has an included angle of about 3°, but the jet is spread by the induction of secondary air so that the actual included angle increases to about 16°. This can be seen from smoke photographs of air jets, the results being summarised in Figures 3.15 to 3.17.

The static regain, or increase in static pressure in the larger duct

Psr =E(Pvi-Pv2) Equ 3.29

Where:

•E = efficiency of conversion expressed as a decimal

It is generally more convenient to calculate the regain from the initial velocity pressure, and to make allowance for the difference by an area term i. e.:

Where: Pv1 Pv2 Ai A2 ■ E |

Initial velocity pressure (Pa) Final velocity pressure (Pa) Initial cross-sectional area of diffuser (m2) Final or outlet cross-sectional area of diffuser (m2) Efficiency of conversion expressed as a decimal |

At 100% theoretical efficiency of conversion: Ps1 Pv1 — Ps2 Pv2 Or Ps2 — Ps1 (Pv1 — Pv2) SO Ps1 Pv1 — Ps1 (Pv1 — Pv2^ Pv2 But |

Pv2=Pv1x|^ |

P„=-E |

Equ 3.30 |

Pvi |

Equ 3.31 |

Figure 3.15 Short diffuser at large angle |

From this a combined factor F may be obtained from the value:

1-1 |

“ ,2′

•F=E

And the regain psr is then Fxps1.

Values of F from experiment are plotted with included angle of A,

Taper and various ratios of Those from the tests of Kratz and

Fellows are the most reliable, see Figures 3.18 — 3.20.

Figure 3.16 Very long diffuser at small angle |

It is impossible to include factors for every possible design of diffuser. Those given are for circular cross-section diffusers. If the cross-section is square or rectangular then the efficiency is somewhat less for a given included angle. It is suggested that an average reduction of 5% or.05 in • E is a suitable allowance.

Diffusers for steel plate industrial fan outlets often transform from rectangular or circular cross-section. Draw the view each way and estimate the mean included angle. Then use the values for a circular design. If the design is critical and has to be passed by a performance test, it is wise to be on the safe side with the factor.

Included angle * Figure 3.18 Diffuser efficiency versus included angle and area ratio |

20 30 40 50 60 Included angle * Figure 3.19 Regain in a diffuser followed by a duct (psr = F x ps1) |

1-1 |

In an exhaust system which has a diffuser fitted on the fan discharge direct to atmosphere, any gain due to this must be subtracted from the calculated resistance of the system. A fan to deal with the required flowrate at this nett resistance is then selected.

If a diffuser follows immediately after a bend in the system, the full recovery will not be achieved. It is then prudent to use about 0.7 x F. With one diameter of straight duct between the bend and the diffuser use 0.8 x F. If 5 diameters of ducts are between the bend and the diffuser then the full values of F as shown on the curves may be used.

Air velocity has some effect on efficiency. When this initial velocity is very high e. g. above 37.5 m/s there is some loss in efficiency but no definite data is available.

Figure 3.20 Regain in an open-ended diffuser (psr = F x ps1) Included angle ° |

The loss of pressure in a diffuser due to imperfect conversion may be calculated by the same method using (1- E) forthe loss factor.

Thus

Psi =(1-E)ps

Equ3.32

It is important to remember that different factors are required for a discharge direct to atmosphere compared to one with a following duct. One should also note that on forward curved bladed centrifugal fans, and indeed on many other modern designs using a shield or tongue piece in its outlet, there is already an allowance for some gain in the catalogue tables or characteristic curves. The listed performance is based upon some regain by expansion from the nett throat area to the area of duct equal to the full discharge connection size. (See Figure 3.21.)

The static pressure is based on readings taken at some distance from the fan outlet and includes this gain. Hence a diffuser cannot add much to the performance. If for reasons of duct design an expander is used, it is customary to ignore any possible gain.

There is a practical limit to the final diameter of a diffuser fitted to a fan outlet if followed by a ducting system. The larger its final diameter, the more expansive is the ducting which follows. It is all a question of economics and the life cycle i. e. initial cost versus running costs. The effect of the diffuser is to reduce the power absorbed by the fan. This saving must be considered in

Figure 3.21 Effect of throat piece and diffusion downstream to full outlet duct area |

Relation to the cost of the ventilation system as a whole, including the fan, motor and ducting system.

On centrifugal fans for mine ventilation a diffuser is invariably fitted. It has a taper on one side only. The discharge is direct to atmosphere, at which point the static pressure above ambient is zero. Thus the static pressure at the fan outlet is negative and as much below atmospheric pressure as the velocity at that point converted into static pressure. A gain in pressure with the final at atmospheric or zero gauge must start from a negative. This negative pressure is transferred to the fan inlet and the fan is selected for the required flowrate at the calculated resistance of the system less the static regain.

Posted in Fans Ventilation A Practical Guide

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