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fatfallenleafElectronics - Devices

Nov 15, 2013 (3 years and 9 months ago)

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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.

ADDITIONAL
QUESTIONS