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AnnouncementsMidterm Project• Prepare groups of 3 to 4 students• You can submit the list at the end of next class
Midterm Exam 03/09/10
- In class Exam: 75 min
be earlier in the classroom
- Example exam posted on the course website
next class: review
Air Handling Units (AHS)
Equipment For AHU:• Fans• Cooling and heating coils• Combustion heat exchangers• Filtration• Dampers, controls, mixing valves• Humidification: steam/spray• Dehumidification: cooling coil/desiccant• Noise control
Air Handling Unit (AHU)
Compressorand Condenser
Roof top AHU
Gas/Electric Heater
to building
Fan
air from building
fresh air
Evaporator
filtermixing
hotwater
coolwater
Return fan
Supply fan
flow control dampers
AHU
Fresh air
AHU schematic
Outdoor air To room
Exhaust From room
Control of air Systems
• Vary temperature, constant flow
• Constant temperature, vary flow
• Vary temperature, vary flow
How does a residential thermostat work?
A. Calculates difference between actual temperature and required temperature and adjusts furnace/AC output accordingly
B. Measures temperature continuously and turns equipment on and off when temperature is different from set point temperature
C. Measures temperature continuously and turns equipment on and off when temperature is outside of specified bounds
D. Calculates difference between actual temperature and required temperature and adjusts the velocity of the fan (flow-rate) accordingly
Constant Temperature, Variable Volume
• On-off systems• Residential and small commercial• Vary volume by turning system on and off
Constant Air Volume
• Single zone constant air volume• Fan always runs
• Vary temperature of air in response to space thermostat
• Single zone constant air volume with reheat• Often used for precise humidity control
• Multizone constant air volume with reheat• Good humidity control, flexible
• Not very efficient
Constant Volume – temperature control
• Proportional control or Proportional Integral control • Thermostat measure: ΔT = T set point – T zone air • Send the signal to the valve of cooling/heating coil
Multizone constant air volume with reheat
55°F
62°F 55°F
Require less
cooling
Variable Air Volume
• Single zone VAV• Varies air flow based on room thermostat
• Multizone VAV• Central chilled air supply
• Zone thermostats control flow to each zone
• Can also use reheat• Still need to provide adequate air flow for ventilation and
for comfort
• Smaller energy penalty than CAV
VAV terminal
Multizone VAV
55°F
55°F 55°F
Vary the flow
Dual-Duct Systems• Can be VAV or CAV• Two plenums with chilled air and heated air• Zone thermostats control ratio Separate duct
for each zone
Summary of Air systems
• Choice between air systems determined by:• Possibility/desirability of reheat• Energy efficiency• Importance of humidity control• Availability of waste heat• Building size
Water systems
• Fan coil• Fan terminal unit
• Baseboard heaters
• Radiant panels, chilled beams
• Two pipe or four pipe water system
or cooling
chilled beams Radiant panel
Radiant panels
Radiant panels
Reading Assignment
Tao and Janis
Chapter 3 Sections: 3.4.1- 3.4.16 (including 3.4.16)
Chapter 6 (from 6.1 to 6.7)
Homework Assignment 4Table 1 Results of cooling load calculation and required amount of fresh air for ventilation
Zone 1Zone 2 Zone 3 Whole
building
Sensible cooling load for zone Time when the max cooling load appear
41000 Btu/hat 4 PM
45000 Btu/hat 6 PM
27000 Btu/hat 8 AM 92000 Btu/h
at 6 PMSensible cooling load for critical hour for whole building (6 PM)
37000 Btu/hat 6 PM
45000 Btu/hat 6 PM
9000 Btu/hat 6 PM
Latent cooling load (internal sources + infiltration)
5400 Btu/h 5200 Btu/h 5400 Btu/h 16000 Btu/h
Required amount of fresh air based on number of occupants
240 CFM(12 occupants)
520 CFM(26 occupants)
240 CFM(12 occupants)
1000 CFM(50 occupants)
Homework Assignment 4
• Supply air 55°F
• Recirculation air 76°F
• Fresh air (Dallas, TX) DBT=102°F