Ductwork Systems and Supply Arch 432

What You Need To Know Become familiar with the materials sizing ductwork Understand sizing units

Ductwork Considerations Shape Size Aspect ratio Velocity Static Pressure of the system Space conflicts

Terms Balanced VAV CV RSH Aspect ratio Velocity Static Pressure of the system

Formulas Btu H / 1.08 (TD) = CFM What is the 1.08? It is the number of Btu H required to increase the temperature of 1 CFM o f air by 1 0 F

Just the Facts One ton of air conditioning capacity is defined as a heat removal rate of 12,000 Btu H Each ton of air-conditioning load usually includes sensible component of 8,000 to 9,000 Btu H. Air handlers typically circulate about 400 CFM for each ton of capacity.

Four Steps for Air Distribution 1) Multiply total tons by 400 to estimate total CFM. 2) Distribute CFM in proportion to floor area. 3) Adjust CFM to reflect anticipated variations in RSH due to specific building conditions. 4) Round off air supply for each room to nearest 25 CFM.

Example A room has a peak winter sensible heat loss of 20,000 Btu H. How much F air must be supplied to maintain room temperature at 70 0 F? The 130 o is off the coils. BTU H/[(1.08)(TD)] = CFM 20,000/[1.08 (130-70)] = CFM 20,000 / 64.8 = = 325 CFM

Example The same room has a peak summer sensible heat gain of 10,000 Btu H. How much 55 0 F air must be supplied to maintain room temperature at 72 0 F? The 55 o is off the coils BtuH/[(1.08)(TD)] = CFM 10,000/[1.08 (72-55)] = CFM 10,000 / = = 550 CFM

Pharmacy Btu H Heat Loss 146,029 Btu H Btu H Heat Gain (Cooling) Sensible 107,200 Btu H Latent 42,339 Btu H

Pharmacy Btu H Heat Loss 146,029 Btu H Btu H/[(1.08)(TD)] = CFM 146,029/[1.08 (130-70)] = CFM 146,029 / 64.8 = 2, = 2,250 CFM Please not that this is one big zone. Need a duct size of 20 x 16 W D

Pharmacy Btu H Heat Gain (Cooling) Sensible 107,200 Btu H Latent 42,339 Btu H RSH/[(1.08)(TD)] = CFM 107,200/[1.08 (72-55)] = CFM 107,200/18.36= 5,839 = 5,850 CFM Please not that this is one big zone. Need a duct size of 24 x 26 W D

Pharmacy Btu H Heat Gain (Cooling) Sensible 107,200 Btu H 107,200/18.36= 5,839 = 5,850 CFM Duct size of 24 x 26 governs Size of Cooling Unit per tons 107,200 / 12,000 per ton = 8.9 or 9 tons

Pharmacy 9 Ton unit x 400 CFM per ton = 3,600 CFM Duct size of 20 x 24

Pharmacy Duct size of 20 x 24 - or - Duct size of 24 x 26 “Velocity pressure is the directional push of an air stream due to its speed” Trost So we trade static pressure and velocity pressure by changing duct size.

Pharmacy Duct size 20 x 24 or Duct size 24 x 26 Static plus Velocity = Total Pressure A decrease in duct size forces air speed and velocity pressure to increase as static pressure decreases. Enlarging a duct will cause air speed and velocity pressure to decrease as static pressure increases.

Facts Insulation ½” 1” or 2” Add two times the thickness of the duct insulation specified. Size return air ducts 2” larger than supply ducts. Reserve 4% of the total building floor area to accommodate air handlers. Reserve 2% of the total building floor area for boilers, chillers, pumps, and electrical switchgear.

Rectangular Lower aspect ratios More energy efficient Use less ductwork Velocity and Area have an inverse relationship to the CFM (CFM = V x A) Velocity and Pressure Drop have a direct relationship Duct dimensions are inside of duct. W D Aspect ratio = W / D 1 to 1

Prof. Kirk’s one-of-a-kind, surefire process guaranteed to result in a mind-numbing law suit.

When Things Go Wrong Mechanical Contractor required to certify that duct plenums were adequately protected

When Things Go Wrong