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Industrial odour control

7 Calculation of outlet height

As mentioned in section 3.3., odour intensity does not decrease proportionate to the odour concentration, and therefore the odour emission should primarily be reduced either by cleaning measures or by changing design and production features. After that odour nuisances can be reduced through dilution via outlets. The odorant emission concentration, C₅₀, before dilution must not exceed 100.000 LE/m³.

7.1 Emission from well-defined outlets

The following calculation is based on the assumption that emission takes place centrally through a well-defined outlet.

The emission height calculations are based on the same formulae as those used in the Danish EPA guidelines No. 7/1974, 3/1976 and 2/1978. However, these guidelines also take account of plume downwash (buoyance or thermal lift).

All formulae use the following consistent units:

The following symbols are used in the formulae relating to odour:

The calculation procedure described below shall not be used in plants with modest odour emission, where the following inequality applies:

R (C₅₀ - C_g) < 100 LE/s

The calculation of the effective outlet height, H_e, in the Danish EPA guideline no. 7/1974, is based on the following formula (LEE and STERN, 1973):

(1):

where Q is expressed in g/s, C_g in g/m³.

In formula (l) the characteristic measurement time for the concentration C_g is approximately 30 minutes. The formula can be used for the average of maximum concentration values of short averaging times ( = l minute), if Q/C_g is corrected. The correction factor 5.5 is chosen on the basis of literature (Gifford, 1960), (Hino, 1968) and (WSL, 1980) in order to assure validity of the method at different H_e.

(2):

The momentum is calculated as:

(3):

Q = RC₅₀

The effective outlet height is calculated from the formula:

(4):

Q is expressed in LE/s and C_g in LE/m³

If the stack is fitted with an efficient jet cap (plume downwash does not occur) the thermal lift, ? H, of the jet cap can be calculated from

(5):

For u the value 4.5 m/s is used, corresponding to typical wind speed in Denmark.

If Vs < 1.5 u (for u = 4.5 m/s: Vs < 7 m/s), downwash will occur and must be considered in the calculation by using

(6):

H_sug = 2d (1.5-V_s/4.5)

for V_s < 7 m/s. If downwash occurs, ? H_sug shall be deducted from H_s in equation (7).

The efflux speed of the gases at the outlet top should always be greater than 10 m/s.

For hot gases or flue gases the plume rise, ? H, should be calculated from

(7):

DH = 0.151 (Rt) ^0,6 H_s ^0.15

The theoretic outlet height, H_s is calculated from the effective outlet height less the greater of either buoyance or thermal lift, and with possible addition in the case of downwash.

(8):

H_s = H_e - DH

If the resulting H_s is more than 80 m, emissions should be controlled by means of construction or design features or by cleaning.

The physical outlet height, H, is calculated from

(9):

H = H_s + h

The outlet height correction value h is, first, calculated according to a) below, and, second, according to b) and the greater of the to values is used as correction value.

a) The correction value h₁ is calculated taking account of buildings within an area limited by a circle with a range of 2H_s from the outlet. Buildings here mean any kind of buildings, and B₁ is the largest difference of level between the outlet base and roof ridge of buildings.

b) The correction value h₂ is fixed at h₂ = B₂ where B₂ is the largest difference of level between outlet base and ceiling in top storey of buildings within an area limited by a circle with an outer range of 20 H_s and inner range of 2 H_s from the outlet. (Buildings here mean any kind of buildings in which people stay for some time (living, office work, production etc.).

Account need not be taken of isolated buildings with a small horizontal extent (< 30^o) at right angles to the outlet and located between 10 H_s and 20 H_s.

outlet

Figure 2:
Values for calculation of outlet height corrections resulting from buildings

7.2 Emission from low sources/open plants

Large quantities of odorous gases should not be emitted from open plants, because the emission can generally not be controlled and/or reduced. The odour emission from open plants varies greatly, influenced by a number of factors such as temperature and wind speed. In practice it is not possible to determine.

New plants, which may be potential odour sources, should therefore in certain circumstances be fitted with covers or hoods enabling treatment of exhaust gases or dilution by means of stacks.

If complaints are raised against odour from existing plants, the odour emission must be estimated roughly, and the decision then taken whether design or production improvements are required and whether hoods and/ or stacks to central outlets should be established.

7.3 Abatement zones

The abatement zone surrounding an odour source can be estimated from the following expressions:

from experience we know that odour can be perceived within a circle with this radius (m) (WSL, 1980), (Miljøstyrelsen, 1982).

Higher outlet

If a outlet height exceeds the value resulting from calculations based on the method described in this guideline. The abatement zone shall be calculated by inserting the effective height of this outlet in the high sources equation. The odorant concentration contribution is, then sufficient to allow later extensions of the plant without new odour abatement measures having to be taken.

Location of abatement zone

If' the abatement zone of a low source is within the factory site, the conditions are satisfactory. However, to make this estimate, emissions from several sources of the same type in the same factory must be added up. If the abatement zones of several sources of the same type overlap each other, the contributions from the sources shall be considered together along the lines specified in (Miljøstyrelsen, 1982), in order to determine the odour concentration contribution, see section 5.2.

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