Category: Technical information associated with Food Industry
Technical Ltd Variable Ventilation Control Gas Isolation Proving
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Food Industry Technical
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.”
Solenoid Valve
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