The dampers straight across are outside air dampers or make-up air coming from an energy recovery unit. According to the mechanical code, commercial buildings require a certain amount of fresh air from the outside to maintain good IAQ for the occupants.
An energy recovery unit allows it to be conditioned before being introduced into the building. Other methods include using enthalpy calculations to open economizer dampers and/or CO2 control of the dampers.
Outside Air Dampers with Energy Recovery - Enthalpy
The dampers at the bottom of the photo are the return air dampers. The return air will mix with the fresh air or outside air and go through the filters where the air is further conditioned based on what the control is calling for based on conditions inside the building, inside the ductwork, and outside ambient air temperature. In HVAC using outside air can also be used for free cooling.
Enthalpy calculations for damper control are made using a humidity and temperature sensor. The building automation system then makes a calculation of an enthalpy value and controls the dampers accordingly. The fresh unconditioned outside air is channeled through the energy recovery unit where the air is conditioned through an energy transfer of heat from the OA to the EA. This gives the fresh air some conditioning before it is introduced into the building through a mixing box that mixes the inside air with the outside air.
Outside Air Dampers with Energy Recovery - Conclusion
Many economizer systems do not have heat or energy recovery as these systems cost more for installation not only for the energy recovery unit but also for the controls to control the system. These systems are generally complex and need the attention of qualified maintenance personnel for proper maintenance and upkeep. As time goes on more and more commercial systems are utilizing energy systems because of the cost of energy.
It is cheaper, in the long run, to have these systems installed rather than paying the monthly higher utility bills for not having these systems in place. The future is energy efficiency and saving as much as possible because the cost of energy will never get any cheaper unless someone can invent cold fusion or a better alternative.
Outside Air Dampers with Energy Recovery