Heating, Ventilating,
and Air Conditioning
Donald Fournier
Adjunct Professor
Dept of Urban & Regional Planning
University of Illinois
Heating and Cooling
Space Conditioning:
~40% of building
energy consumption.
Most of the US has
both heating and
cooling demand over
a normal year.
Quantify using
Heating and Cooling
degree days.
Boilers / Furnaces
•
Boilers
–
Used to heat water
–
either for space heating, providing
domestic hot water or both.
•
Furnaces
–
Directly heat the air.
•
Use boilers in larger structures, furnaces in small structures. Old
boilers generate steam, now hot water.
•
Combustion gases are separated from heating medium.
Firetube
Boiler
Watertube
Boiler
Condensing Boilers
•
Hydrogen in fossil fuels is converted to water by
combustion using significant amounts of the fuel energy.
•
Condensing the vapor, and reducing flue gases to a
lower temperature can get us higher efficiencies.
•
Higher Heating Value vs. Lower Heating Value
•
Issues with doing so:
•
More Expensive.
•
Capture and manage condensate (corrosive).
•
Needs a larger heat
-
exchanger.
•
Flue gases may be corrosive and do not disperse as easily.
•
But condensing boilers can have an AFUE of as much
as 92% whereas non
-
condensing units have an AFUE
of at most 78%.
Condensing Boiler
Furnaces
Electric Heating
•
Electricity is an expensive heat source.
•
Embedded in each kWh of electricity is another two kWh
of heat rejected at the power plant.
•
Building electrical systems provide heat as a by
-
product
(lighting, computing, & plug loads).
•
Older buildings from c. 1950 may utilize electric heating,
due to low first cost and anticipating of cheap electricity
pricing (used to have special low rates).
•
Electric Heating may be viable if used as a backup for
more efficient systems or to avoid significantly large fuel
expenditure
–
small space heaters that allow for
significantly lower overall space temperatures.
Space Heating
•
Three main ways to provide heat:
•
Direct air heating via a furnace.
•
Radiant Heating (electrical or hot water) using
radiators or baseboards
–
generally located near
windows.
•
Indirect heating of supply air through an air handler
and a terminal unit.
•
Well designed systems should heat occupants
and maintain comfort without excessive losses.
Radiation Systems
•
Terminal units either radiators or perimeter
heating convectors.
•
Steam radiators offer poor control.
•
Water radiators can use temperature reset to
control the load. Either may have self
-
contained thermostatic control valves.
•
May also have in
-
floor or in
-
ceiling radiant
heating (tubes placed in the concrete).
Radiant Systems
Tubing and Manifold
Electric Radiant Heating
Hydronic Baseboards
Antique
Radiators
Modern
Radiators
Moving (Pumping) Heat
•
While electricity is expensive for direct heating, it can be much
more effectively used to move heat.
•
Normal application
–
refrigeration, cooling.
•
A/Cs reject heat outside to a higher temperature.
•
Heat pumps do the same thing in reverse.
•
Electricity used to move heat is added to this.
•
Difficulty
–
Performance worsens as temperature gradient
increases, i.e. when coldest.
•
Average COPs of 2.5
-
3, Best case ~6
Absorption Cooling
•
Generally limited to large
industrial / commercial
applications.
•
Not very efficient (COP=<1).
•
Use ammonia/hydrogen or
salt water as refrigerants.
•
Cooling still happens during
the evaporation cycle.
•
Absorption phase equivalent
to vapor
-
compression
condensation.
•
Heat used to evaporate.
Vapor
-
Compression Cycles
•
Use Refrigerants
–
generally HCFCs to transport heat.
•
Compress and condense the refrigerant to release heat
–
at
high pressure
•
Expand and evaporate refrigerant to absorb heat
•
Generally electrically driven but can also be run with steam.
Exchanging Heat
•
Heat Exchangers are used to transfer or
recover heat instead of rejecting it (wasting it).
•
Lots of types / designs
–
wheels, shell and
tube, plates, & heat pipes.
Cooling Towers
•
Evaporate water to cool
–
either air or water.
•
Power Plant / Chiller Heat Rejection.
•
Cools to wet
-
bulb temperature; no compressor needed.
EER / SEER, COP and tons
•
Lots of ways to assess energy efficiency of cooling
systems.
•
Simplest is COP
–
Ratio of Heat moved to work done.
•
Next is EER
–
Energy Efficiency Ratio
–
Cooling in BTU /
Electric Use in Watt hours (Mixed Units!).
•
Conversion
–
EER = 3.41 * COP.
•
SEER
–
Seasonally Adjusted EER.
•
Why? Non
-
uniform load operation with different seasonal
temperatures
.
•
Conversion
–
SEER = EER + 1 to 2
•
1 ton cooling = 12,000 Btu/
hr
of cooling
Window A/C
•
Smallest, least efficient
units are window units.
•
Have low efficiency
components.
•
May be worthwhile if only a
small amount of the space
needs to be conditioned.
•
Generally SEER 9.7
-
9.8
(Energy Star is 10.8)
Split
-
System Units
•
Next level up.
•
Minimum SEER 13 required (30% more efficient
than window units) Can get up to SEER 23.
•
Isolation, improved components, lower noise.
•
Condensing unit often on the
roof
–
Rooftop Unit (RTU)
•
Air
-
based unit
–
uses ducts.
Large Chiller Systems
•
Chilled Water based heat distribution
•
Campus has five chiller plants that
serve a large loop system
•
Utility of large systems
–
most chillers
have a narrow range where they
operate at peak efficiency. Can step
through individual chillers.
•
Larger systems allow for more efficient
cooling towers, and heat recovery
•
In general, reciprocating chillers can
serve the smallest
loads
efficiently,
rotary
-
screw
chillers
are the most flexible, and
centrifugal
chillers are most
efficient
when fully loaded.
Steam Turbine
Chiller
York Centrifugal Chiller
Temperature and Humidity Control
•
Humans have a narrow comfort range for
both temperature and humidity
Ventilation / IAQ
•
Ventilation
–
remove stale air, high in CO
2
, & air contaminants.
•
Outside air is unconditioned
–
needs to be heated or cooled,
have moisture added or removed.
•
At the campus
–
1cfm of ventilated air per year
-
$1.
•
ANSI/ASHRAE guidance for CO
2
–
1000 ppm (IDPH also).
•
Need to filter air as well.
2010
Default
Forced Air systems
•
Ducts, generally in plenum spaces
–
above
ceiling.
•
Larger systems have separate supply and return
fans to move (force) air.
Constant Volume Reheat Syst.
•
Constant Air Volume
-
Most older systems at the University
are this type.
•
A constant volume of air is delivered to a space (with a
variable amount of ventilation).
•
Supply air for the space is cooled and dehumidified to a fixed
temperature (e.g. 55 degrees)
•
Air is then RE
-
HEATED to the necessary temperature for
appropriately cooling/heating the space (cold and warm air
can also be mixed)
•
Fan operates at the design speed all the time.
•
Temperature reset or on
-
off control (small systems) to prevent
simultaneous heating and cooling
Constant Volume System
Dual Duct Constant Volume Syst.
Variable Air Volume
•
Variable Air Volume
–
newer standard system.
•
Heating or cooling provided by varying the volume of
conditioned supply air sent to the space.
•
Air is heated or cooled to a fixed temperature and a
varying amount is provided to that space
•
Amount of air adjusted by a damper
•
Minimum level of air provided to meet ventilation
requirements.
•
Variable speed fan used.
•
Reheat is added to the
perimeter zones.
VAV Reheat System
Displacement and Under
-
floor
Air Distribution
•
These ventilation systems provide
conditioned air at floor level and allow for
natural convection to lift stale air away from
occupants and toward the return air registers.
•
Especially useful in high
-
ceiling rooms and
atria where the entire space does
not need conditioning.
•
Can use under
-
floor plenums
for air distribution.
•
Example
–
BIF
Control Systems
•
HVAC Control systems can use dozens of inputs
–
temperature (interior/exterior), humidity,
occupancy, CO2 levels, schedules, & electricity
prices.
•
Must control motors, fans, dampers, furnaces,
blowers, transducers, actuators …
•
Initial systems were pneumatic
–
highly
simplified control using compressed air.
•
Now HVAC controls are computerized
–
Direct
Digital Control. Even web
-
addressable
Building Automation Systems
BAS
–
Computerized, Intelligent System. Controls Mechanical
and Lighting Systems, Monitors performance, and warns of
system failures
Next Steps
•
Next class we will cover energy conservation
in existing buildings.
•
We’ll take a look at these various systems
and discuss how to make them more
efficiency and use less energy.
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