One of the functions
of an air handling unit is to heat the incoming
airstream. This can be achieved using direct heaters such
as electrical heating elements. However it is more commonly
achieved using heater coils. Heater coils are
composed of a staggered grid of copper pipes conveying
heated water between flow and return headers as
shown in figure. The pipes can be connected by return
bends which allows the flow and return headers to be
at the same side of the heating coil. Coil heat
output is improved by increasing the number of pipe rows.
Low, medium or high temperature hot water or steam
flows through these pipes in parallel assuring
equal distribution of heat across the heater coil face.
Attached to the surface of each tube are aluminum or
copper fins. These fins increase the surface area for
heat transfer between the hot coil and the airflow. Further
increases in heat output are possible by corrugating the
fins, but this does also increases the resistance the
air experiences when passing through the coil.
Temperature control is achieved by fitting a
temperature sensor into the rooms being heated or the extract duct work. This
signal is fed into a control unit and is used to set the position of a valve
supplying hot water from the boilers to the flow header. If the temperature of
air in the extract duct work is higher than the room set point then the hot
water flow to the coil will be modulated down or shut off. Thereby preventing
further unnecessary heating of the room.
