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FIT Variable Ventilation Control Gas Isolation
Proving
Variable Speed Controls for Kitchen Extract
For 95% of the day the ventilation fan is running at full power when
it could be running at a slower speed if at all. The Variable Speed
Drive automatically adjusts the fan speed according to the
conditions in the kitchen detected by optic and temperature sensors.
Using variable speed drives to control your process will save
energy, and save your company valuable costs.
Using a Variable Speed Drive (VSD) to reduce the speed of a pump or
fan application can typically achieve the following savings;
Reduce motor speed by 10%, achieve a 27% reduction in energy
consumption
Reduce motor speed by 20%, achieve a 48% reduction in energy
consumption
The Solution
Temperature and optical sensors are used to detect the cooking
conditions.
Temperature Sensors: Exhaust air temperature is continuously
monitored by sensors connected to the processor that varies the
output of the drives controlling the extract fans.
Low Output: When there is little or no cooking.
Increased Output: When cooking activity increases, the heat sensors
quickly detect this and the extract fan speed is increased to the
appropriate level.
Optic Sensors: These consist of an infrared light beam emitter and
receiver covering the length of the extract canopy. The receiver
contains a photo detector which produces a variable low voltage
signal depending on the amount of infrared light received. With the
presence of smoke or steam, the light received will reduce and on
detecting this, the processor will signal the drive to ramp the fan
motor to maximum speed.
Affinity Laws
The affinity laws are used in hydraulics to express the mathematical
relationship between the several variables; such as head, volumetric
flow rate, speed and power, involved in pump and fan performance.
Fans consume power in proportion to their speed cubed. Therefore an
extract fan running at 50% speed will only consume 13% of the energy
(½ x ½ x ½) but will extract 50% of the air. This is illustrated by
the ‘Affinity Laws for Centrifugal Loads’ as shown above.
Design of Hood Airflow
DW/172 Specification for Kitchen Ventilation Systems
Section 5 - Extract Flow Rates
“The calculation of the optimum extract flow rate is the most
important element of canopy design as too much air will cause as
many problems as to little. Whilst the size of the cooking
appliances determines the size of the canopy to be supplied, it is
the type of appliance that determines the volume of air to be
extracted.”
There are 5 methods of calculation
1 Thermal Convection Method
2 Face Velocity Method
3 Appliance Power Input
4 Air Changes
5 Linear Extract
Although there are 5 methods the only one that should be used is
‘Thermal Convection Method’. Other methods have been included into
the DW/172 for use only when insufficient information is available
at the design stage.
Section 17- Fans
17.8 “Both variable and two speed regulation are now common
features, but it is essential that when grease is being produced,
the system shall. operate at its design duty. A minimum extract
level shall be set within the regulator to ensure that, even when
set at low speeds, an acceptable ventilation rate is maintained as
reduced speeds may cause the carry-over of grease through the
filter. This facility is a standard feature of the controller with
proprietary speed regulators.”
17.10 “BS 6173 states that an interlock of the ventilation system to
the gas supply serving the cooking equipment shall be installed so
that, in the event of air flow failure, the gas supply is switched
off. The supply. fan shall also be isolated when a fire suppression
system is activated in fire mode. This will involve the fitting of
an automatic solenoid valve in the gas supply pipe work and an
airflow-sensing device such as a pressure switch, vane switch or
torque switch.”
“The inclusion of this interlock is a requirement for all new
powered extract. systems. For existing installations where cooking
equipment replacement. is like for like, no action is required.
However, the contractor shall quote for the inclusion at all times;
failure to do so could lead to legal issues. If. an existing kitchen
has any form of upgrade, then the contractor shall. quote for the
inclusion. The Client’s insurance company may have requirements that
this work is done; this may reduce the premium paid.”.
Gas Isolation
BS 6173 British Standard for Gas Isolation
With regards to the above standard it is now a legal requirement to
have a gas isolation system fitted into your commercial kitchen.
The extracts below are the relevant sections taken from the Corgi
Technical Bulletin TB 130:
“If a ventilation (waste gas product removal flue) canopy and
combustion air supply is interlocked to a fail safe (i.e. cuts the
gas off to all flue type gas appliance and any electric supply to
ancillary frying appliances) system that monitors any failure of the
means of waste air being extracted. or choking the combustion air
input, the cooking equipment is deemed. ‘Not to Current standards’
or unsafe.”
“The interlock should have a fail-safe means of shutting off the gas
supply. with a normally closed solenoid valve controlling the whole
supply to the kitchen appliances and ancillary service counters if
they are in the kitchen / compartment.”
“If a Gas Proving System type of interlock control is used, this
means that the gas cannot be turned on inadvertently with any gas
valves open. A safe system and procedure for re-commissioning needs
to be set by. the duty holder, the manager of the chef.”
Food Industry Technical Ltd
Falcon Business Park
Ivanhoe Road
Finchampstead
Berkshire
RG40 4QQ
Tel : 0118 973 9310 | Fax : 0118 973 9311
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