Appendix 4.4 Calculation
of Mechanical Ventilation Rate for the Lorry Parking Area Inside the WFM
1. Introduction
The proposed Wholesale Fish Market (WFM) at Tuen Mun Area 44 will be
enclosed and provided with mechanical ventilation. It will accommodates
sorting, auction and marshalling area for the fish wholesale operation as well
as a lorry parking area where the loading and unloading of fish will take
place. The following section details the determination of the mechanical
ventilation rate required for the lorry parking area inside the WFM to ensure
compliance with the Environmental Protection Department (EPD) air quality
guidelines.
1.1 Air Quality Guidelines
The following guidelines as stipulated in the EPD Professional Persons
Practice Note ‘Control of Air Pollution in Car Parks’ (PN 2/96 issued in June
1996) should be met within the lorry parking area.
|
Pollutant |
Concentration |
Averaging Time |
|
|
|
|
|
a) Nitrogen Dioxide (NO2) |
1,800 mg/m3 300 mg/m3 |
5
min 1
hour |
|
b) Carbon Monoxide (CO2) |
115,000 mg/m3 30,000 mg/m3 |
5
min 1
hour |
For the NO2 and CO are the major constituents of vehicle
exhaust, the build up of these pollutants is the controlling factor of the
ventilation rate required for the WFM.
1.2 Emission Factors
The vehicular emission factors of heavy good vehicles are adopted in the
calculation as given below:
|
Pollutant |
Basic Emission Factors (g/km-veh) |
|
|
Heavy Good Vehicle |
|
NOx |
5.83 |
|
CO |
8.27 |
1.3 Background Air Quality
The ambient concentration for fresh air intake of CO and NO2
is taken from record of EPD's air quality monitoring statistics given in ‘Air
Quality in Hong Kong, 2000’. The 95 percentile of the pollution concentration
measured at Tsuen Wan Distinct was used as tabulated below.
|
Pollutant |
mg/m3 |
ppm |
|
NO2 |
122 |
0.065 |
|
CO |
1490 |
1.303 |
1.4 Vehicular Movement within the
WFM
The worst-case traffic during the peak-hour operation of the WFM was
adopted in the calculation. It assumes that a maximum of 40 vehicles per hour
will be using the parking area.
|
Vehicle Types |
Headway |
Total vehicle movements
per hour |
Idling time per movement |
Total idling time per hour |
|
Lorries |
N/A |
40
lorries |
3
minutes |
120
minutes |
The average speed of vehicle movements is assumed to be 5km/h. The
length of the longest lane of the various types of vehicle is given below.
|
Vehicle
types |
Length
of longest lane inside the car park |
|
Lorries |
85m |
1.5 Conversion Ratio for NOx /
NO2
The conversion factor used for NOx / NO2 is 20%.
1.6 Volume of WFM
The effective volume of WFM is estimated to be 32,109.66m3.
2. Calculation of Ventilation Rates
Using the following formulas (extracted from PIARC 1987), the air
quantity required for the dilution of carbon monoxide and nitrogen dioxide
emitted from the idling and travelling vehicles inside the car park can be
estimated.
Carbon monoxide
Nitrogen dioxide
where:
QF = required air quantity per second (m3/s)
qoCO = basic value of CO emission per vehicle
(g/hr,veh)
qoNO2 = basic value of NO2 emission per vehicle
(g/hr,veh)
Dpc = number of idling vehicles with engine
running
COlim = maximum permissible CO concentration (mg/m3 CO)
NO2lim = maximum permissible NO2 concentration (mg/m3 NO2)
where Mtc = hourly traffic volume of travelling
vehicles, and
v = mean driving speed of vehicles
D = travelling distance (km)
Unit conversion of the air quality limits as stipulated in the EPD's
guidelines are given as follows.
For carbon monoxide:
Molecular weight of CO = 28.01
For nitrogen dioxide:
Molecular weight of NO2 = 46.01
2.1
Idling Vehicles
2.1.1
Carbon monoxide
Ventilation rate required to dilute CO level has been calculated as
shown below.
Lorry idling with engine
running qoCO
= 2 g/min ´ 60 = 120 g/hr =
2.339 m3/sec
* = factor
for ambient fresh air concentration of CO.
2.1.2
Nitrogen dioxide
Ventilation rate required to dilute NO2 level has been
calculated as shown below.
Lorry idling with engine
running qoNO2
= 2 g/min ´ 60 ´
0.20 = 24 g/hr =
74.854 m3/sec
* = factor for ambient fresh air
concentration of NO2.
2.2
Travelling Vehicles
2.2.1
Carbon monoxide
Ventilation rate required to dilute CO level has been calculated as
shown below.
Lorry travelling qoCO =
8.27 g/km-veh ´
5 km/hr =
41.35 g/hr Mtc = 40 veh/hr D = 0.085 km = 0.274 m3/sec
* = factor for ambient fresh air
concentration of CO.
2.2.2
Nitrogen dioxide
Ventilation rate required to dilute NO2 level has been
calculated as shown below.
Lorry travelling qoNO2 =
5.83 g/km-veh ´
5 km/hr ´
0.20 =
5.83 g/hr Mtc = 40 veh/hr D = 0.085 km = 6.182 m3/sec
* = factor
for ambient fresh air concentration of NO2.
3.
Ventilation Rate Required for the WFM
3.1
Estimated Ventilation Rates
|
Source of emissions |
Ventilation rates required for different pollutants |
|
|
CO |
NO2 |
|
|
IDLING |
||
|
Lorries |
2.339 |
74.854 |
|
TRAVELLING |
||
|
Lorries |
0.274 |
6.182 |
|
TOTAL |
||
|
in m3/s |
2.613 |
81.036 |
|
in m3/hour |
9406.8 |
291729.6 |
|
Volume of WFM |
32109.66 |
|
|
Air changes/hour |
0.3 |
9.1 |
3.2
Mechanical Ventilation Requirement
The above calculation results indicate that the total air volume
required for the dilution of nitrogen dioxide will be a dominant factor
affecting the ventilation requirement for the WFM. The minimum ventilation rate
required for dilution of nitrogen dioxide within the WFM is estimated be
9.1Ac/hr. It is therefore concluded that the EPD Air Quality Guidelines for Car
Parks can be met with a ventilation provision of say 10 Ac/hr.