Air today, payback tomo
December 1st 2007 Lenze says frequency inverters fitted to air curtains in the loading bay of a mail sorting
centre is keeping workers comfortable and saving energy – with ROI in under a year
Air curtains are often used where
doors must be frequently opened
or left open. A curtain of high
velocity air either heated or at room
temperature is blown across the door
opening. This causes a barrier to the
natural convection in which cold air creeps
in at ground level and warm air escapes
near the top of the door. Preventing the
loss of warm air and the ingress of cold air
saves energy for the building. Doors can be
left open for the duration of the loading
process and the passage of personnel and
trucks is not impeded.
This massive 160,000m2 mail sorting
centre runs 7 days per week and 24 hours
per day. A total of 72 loading bay doors are
in operation, fitted with hot air curtains to
keep out cold winter weather. It is
estimated that each door is open with the
air curtain running for 3600 hours per year.
The air curtains operate with a
centrifugal fan fitted across the top of the
door, blowing downwards and with
dampeners to direct the flow. Each is
driven by a 5.5kW 4 pole motor. The
required flow is dictated by the size of the
opening and external conditions such as
the wind velocity. In practice these motors
were oversized to cope with the worst case
conditions plus a safety factor – a common
situation with pump and fan drives. The
result in this case was a blast of high
velocity air that seems to bounce from the
floor with turbulence, and permitting a
noticeable ingress of cold air at ground
level. Furthermore the air blast was stirring
up dust. The workers employed near the
doors were unhappy and in some cases
turned the air curtains off.
An investigation by the Plant
Management established the cause of the
problem to be the velocity of the air
curtain, and decided that fitting variable
speed drives was a potential solution.
Three loading bays were to be tested. An
additional decision was whether or not to
use on-site electrical and maintenance
teams for the installation. It was decided to
do so, although it created pressure on other
regular engineering work. The teams were
trained in the products and the necessary
skills to fit them safely, and in fact the
resulting acquisition of skills was viewed
positively by the personnel involved.
The inverters fitted were the new model
SMV from Lenze, a vector drive with IP65
enclosure so that does not require an
enclosure. A RFI filter is integrated into the
inverter, and the SMV features a pluggable
memory chip which allows the inverter to
be set up in seconds, even on the bench
before installation. It is simple to operate
with a keypad and display on the front face.
The engineers at the sorting centre found
the installation without a panel to be easy,
and were able to set it to run whenever the
door is open.
With an outside temperature below zero,
it was found that the temperature in the air
surrounding the retrofitted doors rose from
18 to 25°C. The new temperature was too
high, allowing the air curtains to be turned
down. The motor speed was reduced in
steps to find the optimum according to the
local conditions around the bay doors.
Generally a 30 per cent speed reduction to
35Hz was found to be possible, giving
comfortable working conditions and saving
60 per cent in the consumption of
electricity. The motor power factor
improved by 34 per cent In practice the
energy saving was even greater as the
amount of heat taken from the plant's
heated water system was also reduced.
"Retrofitting a variable speed drive to an
existing air curtain is simple, viable and an
energy saving application that enhances
the comfort and performance of
employees", says the sorting centre's
electrical manager. "The employees are
happy with better working conditions, and
site management gain with productivity
and lower energy costs." More articles from Lenze Ltd: | 
