View Full Version : Air handeling units

30-12-2010, 04:27 PM
Hi I know the princple behind the air handeling unit. But could someone please explain to me how it gets controlled. Does the fan always run or does it switch on and off on demand? What safeties are put in place and why they have got an air flow switch.


30-12-2010, 05:15 PM
Hi Johan2.

I will try to shorten this as much as posible.

A single air handling unit will usually find use in one or more zones. I will try to outline the morden energy efficient set up for you. Please click here (http://www.isentropictemptech.com) if you wish to access my contact page in order for you to contact me if you want me to sent to you more information regarding this.

The air handling unit sucks air from a designated return air space cools it through the chilled wate coil or heats it up through the heater bank. On the discharge side of the fan (SAF) called the supply air side of the AHU somewhere at least three quaters down the suply air duct an air pressure sensor is installed and set at apredetermined design set point. As your rooms or zones start to achieve their set point temperatures the apparatus installed to control room /zone temperature start to close / throttle the supply of air resulting in pressure build up in the supply air duct then a 0-10v signal from the duct pressure sensor is sent to the variable speed drive to adjust the fan speed acordingly. for as long as there is need for air conditioning the fan is not going to stop becase you have fresh air dampers whic must induce 10% fresh air into the building all the time. You also have relief dampers which will have to throw away some air hence circulation must be present at all times. ASHRAE recomends 0.1m/s movement of air around each building occupant hence the fan must always run.

The air flow swich is a safety which will act to cut off power supply to the heater bank in the event that your fan belts snap or your fan does not work for whatever reason. In standard building practices it is mandatory to install fire relays and smoke detectors in the air handling unit. As an extra optional caution an air handling unit can have a door limit switch wired in such a way that if you open the door of the unit the fan and the cooling valve will stop and close respectively.

I hope this will be of use to you, however if you wish to contact me remember to click the link I have given you above.

30-12-2010, 06:29 PM
Temperature is controlled by 3 way propotional water valve in cooling mode and in heating mode, the heating elements are kicked in when temp is dropping or an extra coil for hot water heating or the cooling coil is used for hot water as well.

In small AHU, the temp is controlled by starting and stopping the compressor or the use of inverter.
4 way refrigerant valve is used for cooling and heating.

30-12-2010, 07:45 PM

In addition to the above members comments, a basic system layout.....

Supply air side:-

Fresh air inlet with shut off damper if required
Basic air filter
Frost coil, hot water coil. Safety low temperature thermostat to shut off system if safe temperature not achieved.
Possible air mixing chamber to provide fresh air or recirculated air. Modulating dampers in ducts.
High quality filtration
Pre-heat coil, hot water coil.
Cooling Coil, DX or chilled water, condensate drainage.
Re-heat coil, hot water coil.
Supply air fan
Sound attenuator
Range of ductwork.

Extract fan, draws air from conditioned space.
Connects to supply AHU and also discharges air to outside.


30-12-2010, 09:01 PM
If you are interested, check Carel or Eliwell web cite. They have almost preprogrammed PLC-s for AHU and they have described the system controlls very thorough. AHU are not simple thing, belive me, they have a million of options and accessorize so .....

31-12-2010, 03:08 PM
Thank you for the great explainations guys. I am really learning a lot from you. Enjoy new year. Cheers