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HVAC SYSTEMS
Heating, ventilation, and air conditioning (HVAC)
systems are the lungs of a manufacturing plant. The purpose
of these systems are to supply enough air of the right temperature
and humidity to keep people comfortable and to exhaust harmful or
unpleasant air contaminants. When these systems are maintained
to operate at peak efficiency operating costs can be reduced.
The following module containing the recommendations below attempts
to illustrate the potential savings that can be achieved when these
systems are optimized.
General Rules of Thumb:
- The average cost of electricity is $0.05/kWh
($15/MMBtu)
- The average cost of natural gas is $0.35/CCF
- The average cost of #2 fuel oil is
$4/MMBtu
- There are 2000 hours per year per shift
(based on the assumption that one shift is 8 hours per day,
5 days per week, 50 weeks per year)
- A typical boiler or furnace has
a combustion efficiency of 80%
- Switching from electric heat to
natural gas or #2 fuel oil can reduce heating costs by 78%
Notes:
Before choosing the following targeted recommendations READ THE
FOLLOWING:
Pay back estimates for the following recommendations will use the
equation below. They will vary depending on the, application,
type of installation, and purchase quantity of material and labor
associated with each recommendation. It will be up to the
person doing the analyses to use the URL references below each
equation to help estimate an implementation cost.
The data correlating to
the variables below each equation will be prompted for in order
to execute a calculation. Frequently the fuel cost (FC)
associated with the specific recommendation will be prompted for
in order to calculate the annual cost savings (ACS). Unless otherwise
specific to a particular recommendation the ACS will be calculated
as follows:
| Density
of Air |
at 1 ATM |
| Temperature ºF |
lb/ft3
(weight density) |
| -40 |
0.0914 |
| 32 |
0.0780 |
| 122 |
0.0659 |
| 212 |
0.0572 |
| 302 |
0.0504 |
| 392 |
0.0451 |
| 482 |
0.0407 |
| 572 |
0.0373 |
| 752 |
0.0317 |
| 932 |
0.0277 |
Ref: Fluid Mechanics, Frank M. White, Mcgraw Hill, 1979
- Reduce infiltration
- Recover heat from
waste water
- Recover heat from
refrigeration systems
- Reduce air to be
heated or cooled
- Use destratification
fans to improve air circulation
- Direct hot process
exhaust air outdoors during the cooling season
- Clean or replace
air filters regularly
- Centralize control
of exhaust fans
- Temperature setback
- Use cooling tower
water instead of refrigeration (free cooling)
- Insulation upgrade
or replacement
- Use proper thickness
of insulation
- Reduce window
areas
- Window retrofit
- Route steam lines
to avoid heating air conditioned areas
1. Reduce infiltration
Air leaking through a building envelope can be a hidden sink of
energy. This loss is not always obvious and can lead to an
expensive heating bill.
Close doors and windows in heated or cooled areas;
install sensors controlling roof and wall openings; repair or replace
broken doors and windows; install weather stripping on loose fitting
doors and windows; install door seals, plastic strip doors, or air
curtains at loading dock doors; seal unnecessary dampers, flues,
louvers and other roof and wall openings. The following equation
illustrates the potential savings that can be obtained.
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CFH = average air flow rate through the
opening, CFM (obtained by a velometer traverse of opening, or
estimated using equations for air infiltration in the ASHRAE Handbook,
Fundamentals)
Cp = specific heat of air, 0.24 BTU/lb-Fº
DA = average air density, lb/ft3 (see Air
Density Table)
DT = temperature difference, Fº
HY = hours per year infiltration takes place
DEF = efficiency of strip doors (estimated)
h = efficiency of heating/cooling system (estimated)
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2. Recover heat from waste water
High temperature waste
water can be utilized to provide a medium for preheating a variety
of fluids. Typically recovery of heat from hot domestic
or waste water is exchanged for space heating. The following
equation illustrates the potential savings that can be obtained.
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VF = volumetric flow of hot water, CFM
Cp = specific heat of water, 1 BTU/lb-Fº
DW = weight density of water, 62.4 lb/ft3
T0 = temperature of exiting water,
ºF
T1 = temperature of entering water,
ºF
HY = hours per year hot water is available and can be used
for space heating
h = efficiency of water heating system
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3. Recover heat from refrigeration
The heat discharge from
refrigeration systems is an energy source that can be used to
preheat working fluids in process and heating systems. Depending
on the process or heating demands, location of refrigeration
system, and temperature of discharge, heat recovery may be feasible.
The energy savings that can be derived is as follows.
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TON
= tons of refrigeration
LF = average load factor of refrigeration units
COP = coefficient of performance of refrigeration system
CF = conversion factor from tons to BTU/hr, 12,000
HY = operating hours per year
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4. Reduce air to be heated or cooled
Excess ventilation
due to the neglect of a heating or cooling system can be a energy
sink. Depending on the air quality issue, ventilation systems
should be operated at design conditions.
Reduce ventilation air to a safe minimum
level; reduce building exhausts and thus make-up air; close outdoor
air dampers during warm-up or cool-down periods. The following
equation illustrates the potential savings that can be achieved.
Reduce exhaust --
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CFH = air flow rate, CFM
Cp = specific heat of air, 0.24 BTU/lb-Fº
DA = air density, lb/ft3 Air
Density Table
TA = ambient temperature, ºF
TW = average outdoor winter temperature, ºF (TMY
weather files)
PD = fractional decrease in exhaust operating hours during
heating season
HY = exhaust operating hours during heating season
h = heating system efficiency
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5. Use destratification fan to
improve air circulation
Over head heating, and process waste heat can migrate to the ceiling
area of a plant and create a temperature difference demanding additional
heating of the floor area of a plant This is typical of plants
with ceilings greater than 15 feet. The use of destratification
fans or other methods to improve interior air circulation can reduce
heating costs.
Industry standards for destratification using
10,000 CFM circulating fans show that a fan can destratify 40,000
ft3 of building space in 15 minutes.
The number of fans required is found by FN
= BV/FV, FN is number of fans, BV is building volume,
ft3, FV is volume of air moved per fan.
Destratification will reduce the temperature
at the roof, and thus the heat lost through heat transmission from
the roof.
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U = heat transmission value of roof,
BTU/hr-ft2-Fº
A = roof area affected, ft2
DT = anticipated temperature difference between ceiling
and floor, Fº
HY = operating hours per year
h = heating system efficiency
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6. Direct hot process exhaust air outdoors
during the cooling season.
Process exhaust air
that is at a higher temperature than the plant surroundings during
the cooling season can cause the air conditioning system to cool
redundantly. Typically this exhaust is ducted outdoors or
to a process that demands a fluid preheating. The following
equation illustrates the savings that can be achieved when this
recommendation is considered.
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HP
= compressor horsepower
CF = conversion factor, 2544.4 BTU/hp-hr
ALF = average load factor
CHY = number of hours compressor is used during cooling
season
HR = fraction of output energy recoverable as heat
h = compressor efficiency
COP = cooling system coefficient of performance
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7. Clean or replace air filters regularly.
Filters that are dirty
or neglected can contribute to an undesirable pressure drop creating
a (higher than design) flow demand in a fluid transport system.
This increase in demand can increase energy costs required to
operate the system The following equation illustrates the
potential savings that can be derived when filters are maintained
or replaced.
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CFM
= air flow rate, CFM
SPDc = current static pressure drop across filters, (inches
of water)
SPDa = anticipated average static pressure drop across
filters, (from manufacturer's data), (in. H2O)
K1 = conversion factor, 0.0771 BTU/ft-lbf
K2 = conversion factor, 0.6708 lbf/(ft2- in.
H2O)
HY = operating hours per year
h = efficiency
(ref.- Energy Audits Manual, Government Institutes, Inc.)
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8. Centralize control of exhaust fans
to ensure their shutdown, or establish program to ensure manual
shutdown.
Exhaust fans that are
used for applications other than design purposes can contribute
to energy losses due to fan sizing not meeting the exhaust use.
The control of exhaust systems for their purpose will save on
airflow that would otherwise be wasted If this air flow
is heated or cooled, energy is wasted via the unneeded exhaust.
The following equation illustrates the potential savings that
can be achieved.
Power
Engineering Books
ASHRAE
Thomas Register
CFH
= air flow rate, CFM
Cp = specific heat of air, 0.24 BTU/lb-Fº
DA = air density, lb/ft3 Air
Density Table
TA = ambient temperature, ºF
TO = average outdoor temperature, ºF
RHY = reduction in exhaust operating hours per year
h = efficiency
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9. Temperature setback
Heating and cooling
systems that are not set back during hours of unoccupancy can
be controlled to optimize their energy use. When this is
accomplished an energy savings can be derived.
Keep space temperature lower in heating season,
higher during cooling season, or both; operate HVAC equipment
less; reduce heating level and shut off air conditioning when
building is not in use; install timers and/or thermostats for
heating and air conditioning. The following equation illustrates
the potential savings that can be obtained.
.
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U = heat transfer coefficient for walls
and ceiling, BTU/hr-ft2-Fº
A = area of exterior walls and roof, ft2
Cp = specific heat of air, 0.24 BTU/lb-Fº
DA = density of air, lb/ft3 Air
Density Table
I = infiltration rate during unoccupied hours, ft3/hr
DHS = degree hour savings, Fº-hr/yr (estimated using
U.S.A.F. Bin Weather Data) TMY
weather files
h = average heating efficiency
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- or
CTD = current temperature difference,
Fº
PTD = proposed temperature difference, Fº
CAT = current average indoor temperature, ºF
PAT = proposed average indoor temperature, ºF
AOT = average outdoor temperature for relevant season,
ºF
CEU = current annual energy use for heating/cooling, MMBTU/yr
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or
HTF = building heat transfer factor =
HE/DH
HE = energy used for heating, MMBTU/yr
DH = heating degree hours, Fº-hr/yr TMY
weather files
SBH = total setback hours, hr/yr
DT = setback temperature difference, Fº
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10. Use cooling tower water instead
of refrigeration for preparation of conditioned air (free cooling)
when outside temperatures
are low enough.
During the winter
months process fluids cooled by chillers and refrigeration units
should be diverted to the cooling tower to save energy at the
chiller, however there is an energy increase at the tower due
to the additional time the fan will have to run on the tower.
The cooling tower will have lower operating costs since it will
be using evaporative cooling rather than mechanical cooling.
Controls to switch between free cooling mode and refrigeration
will be required In addition a plate and frame heat exchanger
should be used between the chiller water and the tower water so
that foreign matter will not contaminate the chiller water loop.
The following equation illustrates the savings that can be obtained.
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ASHRAE
Thomas Register
TON = tons of refrigeration
LF = average load factor of refrigeration units
CF = conversion factor from tons to BTU/hr, 12,000
RHY = reduction in operating hours per year
COP = coefficient of performance of refrigeration system
ECT = annual energy used for cooling tower, MMBtu/yr
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11. Insulation upgrade or replacement
Insulation that is neglected
can contribute to an undesirable heat transmission loss or gain
that translates into a higher operating cost of the heating, cooling,
and /or process system.
Install or upgrade insulation on HVAC distribution
systems (hot water pipes, air ducts, etc.). The following
equation illustrates the savings that can be achieved.
12. Use proper thickness of insulation
on walls, ceilings, roofs and doors.
Insulation that is neglected
or of an inadequate thickness can contribute to an undesirable
heat transmission loss or gain that translates into a higher operating
cost of the heating or cooling system. The following equation
illustrates the savings that can be achieved.
Heating season :
-
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Pipe insulation resource
(University of Massachusetts)
A = area to be insulated, ft2
Uc = conductance with no insulation, BTU/hr-ft2-Fº
Ua = conductance with proposed insulation, BTU/hr-ft2-Fº
DHc = number of degree hours below current equilibrium
temperature, (F-hrs)
TMY weather
files
DHa = number of degree hours below anticipated equilibrium
temperature, (F-hrs)
h = heating system efficiency
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Cooling season :
CES1 = energy savings associated
with flux, MMBtu/yr
A = area to be insulated, ft2
Uc = conductance with no insulation, BTU/hr ft2-Fº
Ua = conductance with proposed insulation, BTU/hr-ft2-Fº
COP = cooling system coefficient of performance
DH = number of degree hours above equilibrium temperature
TMY weather
files
CES2 = energy savings associated
with decrease in solar heat gain
AGc = current annual solar heat gain, BTU/ ft2-yr
AGa = anticipated annual solar heat gain, BTU/ ft2-yr
A = area to be insulated, ft2
CES = CES1 + CES2
CES = annual energy savings in the cooling season,
MMBtu/yr
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13. Reduce window areas.
Manufacturing plant building envelopes with large window areas can
benefit from the current window area being reduced.
The primary savings is through a lowered transmission heat loss
through the building envelope. The following equation illustrates
the the potential savings that can be achieved..
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ESt = energy savings due to decreased
transmission heat loss, MMBtu/yr
Ac = current window area, ft2
Aa = anticipated window area, ft2
U = heat transfer coefficient, BTU/hr-ft2-Fº
Ti = inside temperature, ºF
To = average outdoor temperature, ºF
HY = hours per year in the heating season
h = heating system efficiency
NOTE: Some energy savings will also
result from reduced infiltration:
-
-
ESi = energy savings due to
decreased infiltration, MMBtu/yr
DA = density of air, lb/ft3 Air
Density Table
Cp = specific heat of air, 0.24 BTU/lb-Fº
Ti = inside temperature, ºF
To = average outdoor temperature, ºF
NU = number of air changes saved per hour (ACH)
VO = volume of heated air in each ACH, ft3
HY = hours per year in the heating season
h = heating system efficiency
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14. Window retrofit
Manufacturing plant building envelopes with large window areas
can benefit from the current windows being changed to a more insulating
type to prevent transmission heat loss. Old windows, typically
single pane, contribute to a greater heating or cooling load as
compared to multiple paned insulating windows. This translates
into a greater cost to operate the heating and cooling systems.
Use double or triple glazed windows; install
storm windows and doors, translucent window insulation, or plastic
sheets over windows for heating season; reduce summer heat gain
through windows with awnings, trees and shrubs, window tinting
or window shades; add reflective devices on roofs.
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ESi = annual insulation energy savings,
MMBtu/yr (not to be confused with ESi in AR#13)
Uc = current heat transfer coefficient, BTU/hr-ft2-Fº
Ua = anticipated heat transfer coefficient, BTU/hr-ft2-Fº
A = area to be insulated, ft2
Ti = average inside temperature in the heating season,
ºF
To = average outdoor temperature in the heating season,
ºF
HY = hours per year in the heating season
h = heating system efficiency
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15. Route steam lines to avoid heating
air conditioned areas.
Steam lines routed
to provide heating or process steam should be located away from
production areas requiring cooling. A separate space may
need to be constructed to isolate these systems. Once this
is accomplished the cooling load will be reduced thereby decreasing
the energy required to operate the air conditioning. The
following equation illustrates the potential savings that can
be achieved.
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Q
= volumetric flow rate, CFM
d = density of air, lb/ft3 Air
Density Table
Cp = specific heat of air, 0.24 BTU/lb-Fº
T1 = anticipated temperature (without steam lines), ºF
T2 = present temperature, ºF
h = efficiency of boiler
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