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Important variables
Air:
Water:
Conversion:
- Use ASHRAE tables I gave you:
- You can fine them online http://www.engineeringtoolbox.com/unit-converter-d_185.html
(be careful, not all online converters are correct)
Objective
• Finish cooling & energy storage systems
• Define Building Zones
• Learn about air handling and water distribution systems
Absorption Cycle
• Water is typical refrigerant• Strong vacuum - ~0.116 psi or 0.0008 MPa• Boiling point of water?
• Differences from vapor compression• No compressor
• Chemical (e.g. LiBr - hygroscopic) – Absorbs water vapor turns it into a liquid
– Regenerated by removing water
• Requires heat
Absorption CycleStrong vacuum Refrigerant: water Absorber: lithium bromide
Replace compressor
Evaporative Cooling
• Wet media/water spray – direct• Raise absolute humidity, lowers temperature of
conditioned air
• Cooling tower – indirect• Cools through a heat exchanger• Lowers temperature, does not affect absolute
humidity• Can directly cool air in cold weather
Evaporative Cooling
Cooling
Evaporative cooling Compressor Cooling Sorption cooling
DX machine Chiller
Air CooledCondenser
Water CooledCondenser
Convert Cooling machine (DX machine) to heat pumps
Evaporative Chiller
Daily distribution of cooling load
Thermal Storage
• Make ice/chilled water at times of low demand or low energy prices
• Store water in insulated container
• Can be used to reduce overall demand
• Water can be used as secondary water source for fire protection
• Ice/snow
Thermal Storage
Thermal storage
Ventilation Air-flow in Buildings
http://www.epa.gov/iaq/largebldgs/i-beam_html/qref_01_03.htm
Zones In building
• How to define the zone in building
• Based on cooling/heating load requirement
• Based on ventilation requirement
• Based on use of buildings
– different apartments deferent zones
Air Handling Units (AHU) Control of air Systems
• Provide conditioned air• Heating, cooling, humidification, dehumidification
• Ventilation• Provide fresh air, remove contaminated air
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
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
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
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