Heating/Cooling: Interior Zone sequence

Heating

• The interior zones will only have a cooling load and will not require heating.
• The interior zones will be cooling only.

Cooling

• The cold supply air control damper will open and will adjust to maintain a constant down stream static pressure. The recommended static pressure is .25"w.g. The static pressure is field adjustable. Cold supply air will flow to the Vari-Flow diffusers.
• The cooling only Vari-Flow VAV diffuser dampers will position themselves from full open to closed (a field adjustable minimum damper position can be set) to maintain the VAV cooling set-point temperature. The Vari-Flow diffusers will modulate to maintain their individual set-point temperatures

Low-pressure supply air duct design.

We recommend that the low-pressure supply air duct be designed by the regain method of duct design. Maintain constant size duct for the entire run-out. The regain run-out should be sized for the total maximum c.f.m. of air to be delivered to the diffusers from the point of entry of supply air. If the point of entry is at the end of the run-out, then the total maximum volume of air of the diffusers on the run-out is used to size the run-out. When the point of entry of supply air is somewhere along the length of the run-out, then the total maximum volume of each section of the run-out, from the point of entry of supply air to the end of each run-out section, is used to size each section of run-out. Each section of run-out from the point of entry may be a different size duct. Size the duct for an initial velocity of 2000 f.p.m. A run-out servicing 10 diffusers with a maximum air volume of 200 c.f.m. each would require 2000 c.f.m. total. At 2000 f.p.m. and 2000 c.f.m. the duct would be 1 sq. ft. in cross sectional area. The velocity head is used to offset the frictional losses of the duct. This will make the static pressure very stable the entire length of the duct under varying flow conditions. At an initial velocity of 2000 f.p.m., there will be .25" static pressure regain available to offset the frictional losses in the duct. This duct design is very complimentary for use with Vari-Flow VAV diffusers as the total volume of air can be taken off anywhere on the length of the duct. If a Vari-Flow diffuser is below it's temperature set point, and is closed, the unrequired air can be diverted to another Vari-Flow diffuser that is requiring more air. If the same run-out were sized by the equal friction method, the duct would reduce in cross sectional area as air volume decreased along the length of the run-out. Equal friction duct design works best with constant volume systems. If a Vari-Flow diffuser were below set point and closed, the unrequired air would cause the static pressure at the reducing duct transition to increase. The increased static pressure would be used to increase the velocity of the increased volume of air through the smaller section of duct it was being forced through. Further, the increased static pressure would oppose the function of the Vari-Flow diffuser that was trying to throttle down. Not only would the pressure increase but also the frictional losses would increase at the reducing transition from increased velocity and turbulence. Equal friction designed duct is not complimentary for use with Vari-Flow diffusers and requires more cross sectional area and consumes more valuable plenum space. Equal friction designed duct also costs more in materials and labor to install (i.e. more pounds of metal). Consider that straight sections of constant size duct are easily fabricated by machine and easily assembled. With equal friction each reduction in duct size requires an expensive reducing transition and new set up for the different straight duct sizes.

NEXT: More on Interior Zone sequence of operation.