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