FLUE DILUTION FANS

Flue Dilution Fans

With the main advantage of avoiding the use of unsightly or expensive flues, regulations require that if the products of combustion are dispersed at low level, then the CO2 content must be 1% or below. The Airflow flue dilution fans achieve this by introducing fresh air into the boilers discharge flue duct and diluting the flue gases. With the correct selection of fan for the boiler rating and the recommended installation, mixing of fresh air and combustion gases takes place before the fan inlet, within the fan and in the discharge section giving satisfactory and safe discharge at low levels. Correct installation also ensures a long and trouble free service life. However, on the rare occasion that a problem may occur with the fan or in the flue duct system the built-in vane switch is an important safety feature. The vane switch will cut out if the fan stops operating or if the duct system becomes blocked, thus shutting down the boiler. For normal operation this switch allows the boiler to fire when the fan is operating correctly and dilution air is entering the fan.

The Range

The airflow range of flue dilution fans is avaliable in 5 sized to satisfy the dilution needs of industrial and commercial boilers rated up to 650 Kw (2,200,000 Btu) input. Each size is avaliable in standard form (GBDF series) for atmospheric boilers and water heater of circa 75% efficiency. Enhanced corrosion resistance versions (SSDF series) with stainless steel fan cases are also avaliable for installation where regulations or the specification calls for stainless steel ducting, and, when higher efficiency boilers such as modular designs are likely to produce condensation.

Typical Installations

IMPORTANT - When designing and installing a dilution system incorporating Airflow dilution fans attention should be paid to the latest edition of the following standards and guides.

(i) BS6644 - 1991 Installation of gas fired hot water boilers of rated input between 60Kw and 2Mw.

(ii) British Gas Corporation Publication IM11 flues for commercial and industrial gas fired boilers and water heaters.

(iii) Department of Environment - Chimney heights, Third Edition of the 1956 Clean Air Act Memorandum. The boiler is connected by a vertical flue to a header which is open to the outside air at both ends. One end of the header acts as the primary air intake for the dilution air and the other as the discharge. The fan is located on the discharge side of the header duct.

Note:- A draught stabiliser or diverter must be incorporated in the boilers primary flue, if not part of the boiler. Shown below is a typical boiler house installation incorporating an Airflow dilution fan illustrating the requirements for satisfactory and safe operation.

 

Choosing the Correct Size and Type of Fan

If possible there should be at least 2 metres of flue ducting from the fan to the outlet. To ensure a maximum of 1% CO2 content at the outlet the volume flow rate of diluted flue gases necessary for a given boiler can be calculated as follows: Flow rate in litre/s = 2.69 x rated input of boiler in kw Where 2 metres of discharge ducting is not possible then the calculation is: Flow rate in litre/s = 4.44 x rated input of boiler in kw The volume flow rate provided by the fan will depend on the static pressure imposed by the size and length of flue ducting and the number of bends, louvres etc. comprising the installation. The performance table below enables selection of the correct dilution fan based on the flow rate requirement and the fans ability to overcome duct system resistance. (Note if LPG or Butane are being used then the factors above should be increased to 3.23 and 5.33 respectively) The flue dilution fans must not be used for other fuels.

Typical Performance Table at 20c The above calculation should generally be used. However, if all the following conditions can be met then the typical performance table shown below can be used.

(i) Ducts match inlet and outlets of fan

(ii) The louvres on the discharge and fresh air inlets have 50% FREE areas and not less than the minimum sizes shown in the chart below.

(iii) The dilution of air inlet is on the same face of the building as the discharge. (iv) There are not more than two right angle bends in the system.

(v) The flue system is not more than 10 metres total length and includes not less than 2 metres after the fan outlet.

 

 

 

General Installation Advice

Each Airflow flue dilution fan is provided with a fresh air inlet on the drive side of the fan. Typically 30-40% of the air passing through the fan enters this inlet which if not ducted to the outside takes fresh air from the boiler room. An adequate supply of fresh air by means of inlet ouvres in the boiler room walls must be made avaliable to the boiler to provide air for combustion and ventilation. To ensure this and provide dilution air to an iducted motor side inlet the following calculation for high and low levels grills should be used. Low level (intake) 1080cm2 plus 9cm2 for each kilowatt input in excess of 60kw rated input. High level (extract) 270cm2 plus 2.25cm2 for each kilowatt input in excess of 60kw rated input. In order to avoid recirculation and therefore build-up of products of the combustion in the system, it is advisable to ensure that the dilution air inlet is at least 2 metres (6ft) from the discharge. If positioning of discharge and inlet is difficult, it is always better to have the discharge higher than the inlet as the warm diluted products of combustion will rise due to convection and be taken away. If the inlet and discharge are well over 2 metres (6ft) from each other their height relationship is generally of little consequence. It is essential to provide easy access to the fan motor to facilitate repair or maintenance. (see minimum clearence figures opposite.)

Condensation

It is recommended that drains are incorporated into the duct systems when condensation of the combustion products is likely to occur (see typical installation program). Condensation should not be allowed to collect in the fan casing.

Electrical Installation

In all classes of installation, it is essential that the fan flow switch is connected into the supply circuit of the appliance gas valve so that the gas valve is cut off in the event of a fan failure or flue system blockage. After the fan has been installed and electrically connected, a check should be made to ensure that the fan flow switch causes the boiler to be switched off when failure or blockage is simulated.

Approval of System

Boiler installations incorporating fan diluted flue systems with low level discharge are subject to the conditions of the Clean Air Act and therefore local Environmental heath Approval should be sought.

Capacity of Vane Microswitch

The switch has a non-inductive capacity of 5 amps at 240V ac and this is considered to be sufficient for appliances fitted with pressure jet burners, the burner and the gas valve must be isolated simultaneously. In this case the vane microswitch may have insufficient capacity for this function and a relay system must be introduced in the circuit between the vane microswitch and the pressure jet burner system. This relay must have sufficient capacity to meet the appliabce manufacturers current ratings.

Specifications

 

Minimum clearance for servicing motor and vane switch (between motor side inlet and any obstruction).

 

Materials

Materials Standard GBDF units have cases manufactured from Aluzinc coated mild steel, with cases finished in black polyester high temperature paint. Enhanced corrosion resistance SSDF units have cases manufactured from 316 grade stainless steel and are supplied in this finish. Impellers for the GBDF and SSDF sizes 2 and 3 are manufactured in aluminium, while the sizes 4, 5 and 6 of both versions are produced from Aluzinc coated mild steel for improved resistance.