Automated Environmental Control Systems In Composting And Mushroom Growing

Air Handling Unit for Mushroom Growing

Air Handling Unit (AHU) is the most important part of environmental equipment used for growing mushrooms. Mushrooms are like humans, they consume O2 from air and produce CO2. We need to supply enough air to mushrooms to let them breathe as well as effectively remove CO2 away from them. Besides providing the mushrooms with air, we need to dry or wet, to chill or heat the air depends on outside climate conditions and growing subphase. All of these functions should be fully provided by AHU with set precision. When ordering a control system for the growing rooms, the grower should understand that even the most advanced control system without properly designed AHU is just a wasting of money. AHU design for growing mushrooms is different to standard AHU used for office or industrial HVAC applications. 

Buyers of TERA’s control system receive calculations and design requirements for air handling units, air ducts and air nozzles, room design and rack layout free of charge.

The basic principles of AHU for growing mushrooms is shown on the diagram below.

Inlet radial fan under control of VFD (variable frequency drive or frequency inverter) supplies the required volume of air in the growing room. We recommend buying VFD with local warranty as it’s a very expansive and important part of the AHU. The fan is connected to AHU with flexible insert as air can have a temperature up to 75 C Degrees and can be humid during steaming process.

The air is heated by a hot water coil or chilled by cold water coil depending on the outside temperature and mushroom growing subphase. It’s a big mistake when some inexperienced growers from countries with hot and wet climate remove or do not order heating coil. In this way they disable the drying function of the AHU which is highly important for growing in their climate.

Hot and cold water valves are controlled by analogue actuators. We recommend to use Belimo valves and actuators with 0-10V output and 2-10 V feedback signal.

The volume of fresh air supplied in the growing room is controlled by the opening degree of the fresh air (FA) damper. Recirculated air (RA) damper operates in antiphase according to position of FA damper. Both air dampers are controlled by actuators. It’s possible to use one actuator for FA and RA dampers with mechanical arm. The AHU must contain a glass window in the mixing chamber for visual inspection of the air dampers.

The exhaust axial fan is used to increase possibilities of the inlet fan to remove CO2 from the room and keep small positive air pressure inside the room. Using exhaust fan is recommended on rooms with 35 tons of loading and more. Exhaust fan is placed on the opposite wall to the entrance of the room and is controlled by its own VFD. 

The section of cold water coil contains a special droplet eliminator for collecting condensate.

The bottom parts of the mixing chamber and cooling module are made of stainless steel and have a special design to drain the condensate through a siphon.

The whole unit is mounted on a stainless steel tray.

The inner surface of the AHU must have corrosion resistant paint or coating or made of stainless steel. All inner connections must be properly sealed. No air leakage is allowed.

Basic AHU design diagram

1 - Air filter
2 - Water coling coil
3 - Droplet iliminator
4 - Fresh air damper

5 - Recirculated air damper

6 - Window in the mixing chamber
7 - Water heating coil
8 - Flexible connection

9 - Radial inlet fan
10 - Stainless steel tray with siphons for condensate drainage