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Chapter 17
Ventilation Operating Costs
A. Fan Efficiency and its Operating Costs
B. Electricity Costs for Fans
C. Make-up Air
D. Heating Costs for Make-up Air
E. Cooling Costs for Make-up Air
F. Energy Conservation
17: Ventilation Costs 1 printed 09Jul08
The learning goals of Chapter 17:
G Know how to use the nominal cost of $100/year per 1000 cfm per "wg
to estimate the fan operating costs for LEV systems.
N Understand the concept of heating and cooling degree days and how
they are used to estimate the make-up air conditioning costs.
N Be familiar with the differences in heating costs among heating sources
and between heating and fan electrical costs.
Be able to use either Eqn. 17.9b (with Table 17.1) or 17.11 (with Table
17.2) to estimate the make-up air heating costs at a given flow rate Q.
Be able to use the "thermal coefficient table" (Table 17.3) to estimate
the cooling costs at a given flow rate Q.
17: Ventilation Costs 2 printed 09Jul08
Only the "operating cost" portion of local exhaust ventilation is covered
in this chapter.
"Capital" Labor Operating
Hardware purchases T
System installation T
Regulatory compliance permits T T
Employee education/training T T
Fan electricity T
Heating and cooling T
Maintenance T
Waste handling, recycling, disposal T
17: Ventilation Costs 3 printed 09Jul08
Fan Efficiency and Power Consumption
1. Fans have a mechanical efficiency …
"power" = energy / time
power imparted to air
%mechanical efficiency = ))))))))))))))))) × 100 Eqn. 17.1
power consumed by fan
Energy Vol Energy
2. Power imparted to air = FanTP × Q = ))))) × ))))) = )))))
Vol minute minute
P = "energy per unit of volume" (even P as head) from Chap. 11.
Q = volume per minute (as in cfm or m³/s).
1 horsepower [hp] = 33,000 ft-lb/min
hyphen
FanTP ["wg] × Q [ft³/min]
hpimparted = ))))))))))))))))))) at 70°F & normal P Eqn. 17.2b
6356
17: Ventilation Costs 4 printed 09Jul08
3. Power consumed by the fan is usually from electricity that is measured in
units of kilowatts [kw], although "hp" is also common in U.S. industry.
amperes × volts × "phase factor"
kilowatt = ))))))))))))))))))))))) Eqn. 17.3
10³
1 kW = 1.34 hp
phase factor (a.k.a. "power factor") =
= 1 for plain resistance (as in a DC circuit).
< 1 for "impedance" in an alternating circuit (AC).
Peak current lags peak voltage for an inductive load.
Peak current leads peak voltage for a capacitive load.
Energy = power × time e.g., kWhr for electricity (coming up) …
17: Ventilation Costs 5 printed 09Jul08
Three potentially useful equations, but none of which are boxed …
Q × FanTP × 100 Q × (FanSP + VPoutlet) × 100
%mechanical efficiency = )))))))))))) = )))))))))))))))))))
6356 × hp 6356 × kW × 1.34
Q × FanTP × 100
hpconsumed = )))))))))))))))) for fan motor Eqn. 17.4b
6356 × %mechanical efficiency
Q × FanTP × 100
kWconsumed = )))))))))))))))) for fan electricity Eqn. 17.4c
8523 × %mechanical efficiency
17: Ventilation Costs 6 printed 09Jul08
Electricity (Operating) Costs for Fans
1. Electricity is purchased in units of kWhr (maybe with a "phase factor").
kWhr = [kW or W/10³] × [time in hours].
Rates vary. $0.07 per kWhr was typical in 2006.
e.g., see www.eia.doe.gov/cneaf/electricity/epav1/fig12.html
2. Energy costs can be predicted …
Energy Cost = (power) × (usage) × (energy price)
Eqn. 17.5
$ = kW hours $ / kWhr
17: Ventilation Costs 7 printed 09Jul08
Example #17.2: Estimate $/yr to operate the fan for TiO2 LEV in Chap. 14.
Q = 1370 cfm
FanSP = 1.30 "wg
FanTP = FanSP + VPout (3867 fpm = 0.93 "wg) = 2.23
assume fan efficiency = 70% (typical for a radial fan).
a. Find the size (power) of the fan motor …
Q × FanTP × 100 1350 × 2.23 × 100
kW = ))))))))))))) = )))))))))))))) = 0.50 kW
8523 × %mech. eff. 8523 × 70
b. Electrical amperage = Watts / (Volts × "phase factor")
= 500 W / (120 V × 0.95) = 4.4 amps
Amps = 500 W / (220 V × 0.95) = 2.4 amps
c. Cost for continuous usage (24 hr/day×365 days/yr = 8760 hr/yr) …
$ = (8760 hr) × (0.50 kW) × (0.07 $/kWh) = $307 / year.
Cost for 8 hour/day business hours = ~2250 hrs or ~25% of 8760 …
$ = (2250 hr) × (0.50 kW) × (0.07 $/kWh) = $79 / year.
17: Ventilation Costs 8 printed 09Jul08
3. A convenient benchmark
for a 70% efficient fan in continuous-operation …
° 1000 cfm at 1 inch FanTP for 0.07 $/kWhr costs about $100 per year
This cost is scalable on the basis of either Q or/and FanTP.
"Scalable" means costs are proportional to another Q or/and FanTP.
An example of using this "rule-of-thumb" …
What is the approximate cost per year
to move 100,000 cfm at a FanTP of 4 "wg?
17: Ventilation Costs 9 printed 09Jul08
Using the above "scalable" rule-of-thumb …
Q FanTP
Fan energy $ . $100 × ))))))) × ))))))
1000 cfm 1 "wg
100,000 4 "wg
Fan energy $ . $100 × ))))))) × ))))))
1000 cfm 1 "wg
Fan energy $ . $40,000 per year
17: Ventilation Costs 10 printed 09Jul08
Make-up Air
Over time, the make-up air flow rate = exhausted air flow rate.
The make-up air should flow through a building's general ventilation
system, cf., "infiltration" (see Chap. 19-20), unless the building has
many permanently open windows or/and doors.
Uncontrolled fresh air ("infiltration") is untempered (as hot or cold as
outside) and unfiltered (dust may be a nuisance or degrade product quality).
1. Locate the make-up air supply inlet to avoid bringing in contaminants …
• from building exhaust stacks (re-entrainment in Section 15.VI).
• from local external sources of contaminants (e.g., loading docks,
smoking areas, waste storage).
• from sabotage or terrorism (e.g., avoid ground-floor air inlets). See
"Guidance for Protecting Building Environments from Airborne Chemical,
Biological, or Radiological Attacks." NIOSH Pub No. 2002-139 (pdf).
17: Ventilation Costs 11 printed 09Jul08
2. Resistance to the flow of make-up air
into a building will increase the
total losses for an LEV system.
If a new LEV system is added to a
"tight" building,
either the flow of fresh air into that
building should be purposely increased
or the RPM of the exhaust fan should be increased to compensate for the
additional pressure loss to get the make-up air to flow into the building.
Any "resistance" to the flow of make-up air that results in a "negative
building" (like a negative "SP") can create problems for occupants …
0.01 - 0.05 "wg impairs the normal flow of short thermal-draft chimneys
(like on a small hot water heater emitting CO).
0.03 - 0.1 "wg starts to impair the normal flow of propeller fans
(like a small wall ventilator or bath room fan).
0.1 - 0.25 "wg starts to affect the normal flow of centrifugal fans, can cause
back-flow in larger chimneys (e.g., a furnace or boiler), and
creates an inconvenient force on doors (10-25 lb).
17: Ventilation Costs 12 printed 09Jul08
Water traps (often called a "p-trap") are installed in plumbing to block the
upward flow of sewage odors.
The water can evaporate from unused drain traps, e.g., in floors.
Even small negative building pressures can draw sewage gases up
through a dry water drain trap.
Negative pressure surges can slosh
some of the 1.5 - 2 " of water out
of a sewage water trap and thus
reduce its ability to prevent
the back-flow of sewer gases,
resulting in odors inside the building.
3. OSHA has only a couple of standards on
make-up air
e.g., 1910.94(c)(7) for spray finishing operations
and 1910.124(d) for dipping and coating operations.
17: Ventilation Costs 13 printed 09Jul08
Heating Costs for Make-up Air
1. Heaters have a thermal (heating) efficiency …
Heat imparted to make-up air
% thermal efficiency = ))))))))))))))))))))) × 100 Eqn. 17.6
Heat consumed by the heater
power imparted to air
cf., %mechanical efficiency = ))))))))))))))))) × 100 Eqn. 17.1
power consumed by fan
Thermal efficiencies vary among heating systems …
a. Direct fired furnaces.
b. Indirect fired furnaces.
c. Heat pumps (essentially a refrigeration system run backwards).
17: Ventilation Costs 14 printed 09Jul08
a. Direct fired furnaces release all combustion products into the heated air.
Advantages …
• generally nearly 100% efficient.
• require no chimney.
Disadvantages …
• release combustion products indoors (e.g., H2O, CO, maybe HCl).
• limited to burning gaseous fuels (natural gas or propane).
• usage may be regulated locally.
b. Indirect fired furnaces keep the combustion products separate from the
heated air and exhaust the combustion products outdoors.
Advantages …
• combustion products are vented outdoors.
• can use oil (even coal) as well as gaseous fuels.
Disadvantages …
• large units are generally less efficient than direct fired.
• require a chimney.
c. Heat pumps can be equivalent to 200-300% efficient (see 17.V.2).
17: Ventilation Costs 15 printed 09Jul08
2. Heat imparted to air can be calculated either by a change in its enthalpy
or by using a specific heat times a change in its temperature.
Enthalpy is the total energy content of a parcel of air that includes …
"Sensible heat" associated with the air's temperature and
"Latent heat" associated with its humidity.
Specific heat is the energy needed to change the temperature of one unit
mass (e.g., kg, lb, or ft³ at NTP) of a given substance by one degree.
• The specific heat of air varies slightly with temperature, significantly
with its absolute humidity, but doesn't apply if the humidity changes
via evaporation or condensation.
• The specific heat of dry air approximates a constant value. Cold air is
dry. So, heating costs can use just one specific heat as long as the
make-up air is not humidified.
• The specific heat of humid air varies. Hot air is often humid. So,
cooling costs depend upon the local climate.
17: Ventilation Costs 16 printed 09Jul08
Annual Heating Degree Days [AHDD or "dg" in the Manual]:
Each day's "heating degrees" = the number of degrees that the time-
weighted average daily outside air
temperature is below 65°F.
A "traditional" building is nearly in thermal balance when
the outside air is about 65°F.
An average daily outside temperature > 65°F adds zero HDD.
AHDD is the averate of the sum of all the heating degree days for a
given location for one year.
AHDD from the NOAA, National Climatic Center are in Appendix C.
The AHDD in the Wasatch region are 6000-7000 °F×days/year
as well as in most of CO, NE, the mid-west, and New England.
17: Ventilation Costs 17 printed 09Jul08
3. AHDD can be used to estimate the average amount of heat that needs to
be imparted to the make-up air at a given location to heat it to 65°F.
Btu/yearheating = [flow rate] × [specific heat] × [AHDD] × [fraction run]
ft³ Btu °F day 1440 min hours/week
ft³ °F — year — 168 hrs/wk
= Q ))) × 0.018 )))) × AHDD ))))) × ))))))) × ))))))))
min day
hrs/wk
Btu/yearheating = Q [cfm] × 25.9 × AHDD × ))))) Eqn. 17.7
168
Btu min
)))))))
ft³/°F/day
17: Ventilation Costs 18 printed 09Jul08
Example #17.3: Find the annual energy that must be transferred to heat
1000 cfm continuously where AHDD = 6500 °F days.
hr/wk
Btu/year heating = Q × 25.9 × AHDD × )))) Eqn. 17.7
168
Btu/year heating = 1000 × 25.9 × 6500 = 168×106 Btu/yr
45 hr
If the fan only ran 5 days/week, 9 hours/day = ))))) . 25% of above.
168 hr
45 hr/wk
Btu/year heating = 1000 × 25.9 × 6500 × )))))) = 45×106 Btu/yr.
168
17: Ventilation Costs 19 printed 09Jul08
Heat imparted to air [Btu/yr]
4. Heat consumed = )))))))))))))))))))) Eqn. 17.8
heater % themal efficiency / 100
5. Heating costs = Heat consumed × [price/Btu].
Finding the $/year the "long way" always works …
Eqn. 17.9b
Heating costs in Equation 17.9 vary based on …
a. Q [cfm] being exhausted (equals make-up air being conditioned).
b. AHDD [°F×days/year] (varies with the location's climate).
c. Heater efficiency [%] (varies with the type of fuel used).
d. Fuel's energy content (units vary with the fuel).
e. Price of the fuel (varies with the energy market).
f. Fraction of time the exhaust system runs (varies with the use).
17: Ventilation Costs 20 printed 09Jul08
Table 17.1 contains the typical costs per 106 Btu/year consumed …
add to Book
6 106 × Price [$] per unit of fuel
$/10 Btu = ))))))))))))))))))))))))))))) Eqn. 17.10
Btu/unit of fuel × Heater efficiency / 100
Coal Natural Gas Fuel Oil Electricity
Price of fuel 45 $/ton 0.005 $/ft³ 1.00 $/gal 0.07 $/kWhr
Energy Content 13,000 B/lb 1000 B/ft³ 140,000 B/gal 3413 B/kWhr
heat. efficiency - 45 % - 90 % - 75 % - 100 %
Cost per 106 Btu 3.85 $/MBtu 5.56 $/MBtu 9.52 $/MBtu 20.51 $/MBtu
Finding the $/year the "short way" works only if the above rates apply …
[Btu/yr]
Annual $ for heat = ))))))) × [Cost per 106 Btu] Eqn. 17.11
[106 Btu]
Cost per 106 Btu may be obtained either from Eqn. 17.10 or Table 17.1.
17: Ventilation Costs 21 printed 09Jul08
Example #17.4:
Calculate annual cost to heat 1000 cfm with natural gas in a 6500 AHDD
environment (such as in northern UT or OH, IA, IL, IN, MA, NE, NY).
The most general solution is to use Eqn. 17.9 (the "long way") …
Eqn. 17.9b
1000 × 25.9 × 6500 0.005 /ft3 168
Annual $ = )))))))))))))) × ))))))) × ))) = $935 Eqn. 17.9
90 / 100 1000 B/ft3 168
If you are given the Btu/yr (or you calculate it as in Example #17.3), then
Eqn. 17.11 is quicker …
Annual $ = 168×106 Btu/yr × 5.56 $/106 Btu = 935 $/yr Eqn. 17.11
17: Ventilation Costs 22 printed 09Jul08
6. A comparison of annual heating costs among fuels and to the fan …
Table 17.2 Annual costs for 1000 cfm in 6500 AHDD climate.
Continuous 25% of the time
Heating via …
electricity $ 3445 $ 861
fuel Oil $ 1600 $ 400
natural Gas $ 935 $ 234
coal $ 650 $ 163
Electricity just for
the fan motor $/yr + $ 200 + $ 50
at 0.07 $/kWhr
and FanTP = 2 "wg.
The above heating costs are "scalable" with Q within this climate
(not quite a rule-of-thumb because it is explicit to one climate).
17: Ventilation Costs 23 printed 09Jul08
Cooling Costs for Make-up Air
Like heating costs but with two complications and one simplification …
• The specific heat of air increases (by ~ 15%) with its moisture content.
• Condensation of humidity changes "enthalpy" too much to be useful.
• Virtually all cooling systems are driven by electricity. (
1. Refrigeration takes heat from a warmer to a cooler temperature.
Figure 17.2 depicts a typical two-phase vapor compression cycle …
• A compressor.
• A refrigerant (circulates).
• A condensor and external fan
(dissipates heat into warm-air).
• An expansion valve.
• An evaporator with another
external fan (absorbs heat
from cold-air).
17: Ventilation Costs 24 printed 09Jul08
2. Coefficient of Performance [COP] is used in cooling instead of
thermal efficiency (% thermal efficiency).
Heat transferred from the air EER
COP = )))))))))))))))))))) = )))) Eqn. 17.12
Energy consumed in system 3.414
EER ("energy efficiency ratio" used by U.S. EPA) also converts Btu's
transferred (thermal units) into Watts consumed (electrical units).
The COP in a good cooling system should be between 4 and 6.
The COP in a room air conditioner or a pre-1990 system might be ~ 2.
A heat pump is a refrigeration system run for the reverse reason : it
takes heat out of cold (outside) air and pumps it into warm (room) air.
Both a refrigeration system and a heat pump
typically have a thermal efficiency > 100%
(they move more heat than they consume).
17: Ventilation Costs 25 printed 09Jul08
3. Annual cooling degree days [ACDD] are similar to AHDD.
Each day's "cooling degrees" = number of degrees that the time-
weighted average daily outside air
temperature is above 65°F.
ACDD is the average of the sum of all the cooling degree days for a
given location for one year.
Enthalpy is preferred over specific heat to calculate the amount of energy
needed to cool air because …
• the humidity of hot air can vary so much more widely than cold air
and
• condensation (at the air's dew point) affects air's enthalpy so greatly.
17: Ventilation Costs 26 printed 09Jul08
Figure 17.3 depicts a
psychrometric chart to which
lines of constant enthalpy
(Btu/ft³) were added
(dashed lines that nearly parallel
the wet bulb temperature lines).
Cold winter air (lower left)
is always dry, has a
nearly-constant specific heat,
and heating never causes
condensation.
The specific heat of hot summer air (right side of chart) depends upon both
its temperature and its moisture content or humidity (viz., its dry bulb and wet
bulb temperatures).
17: Ventilation Costs 27 printed 09Jul08
Two enthalpy equations adapted from McQuiston and Parker (1988) …
Enthalpy [Btu/ft3] = 0.01775×°F +
Enthalpy [kJ/m3] = 1.185×°C +
where Pair is the ambient barometric pressure [mmHg] and
Pwater is the partial pressure of water vapor [mmHg].
… were used to calculate equivalent "thermal coefficients" in Table 17.3 to
allow Equation 17.14c (used to calculate cooling) to "look" very much like
Equation 17.7 previously used to calculate heating …
fraction of
Btu/year cooling = Q × "thermal coefficient" × ACDD ×
time running
was 25.9 Btu min/ft³/°F/day
in Eqn. 17.7 for heating.
See values in Table 17.3 for cooling.
17: Ventilation Costs 28 printed 09Jul08
4. The "thermal coefficients" in Table 17.3 can be used in Equation 17.14c
or 17.15a to estimate the change in enthalpy needed to cool air starting
from a locally typical temperature and humidity range to 65 °F.
fraction of
Btu/year cooling = Q × "thermal coefficient" × ACDD × time running
Table 17.3 "thermal coefficients" to cool make-up air to 65 °F.
typical dry (< 30% RH) moderate (30- 60%) humid (> 60% RH)
Outside T ")E/)T" "therm.coef." ")E/)T" "therm.coef." ")E/)T" "therm.coef."
100 °F 0.0179 25.8 0.0308 44.4 0.0665 95.8
90 °F 0.0179 25.8 0.0194 27.9 0.0552 79.5
80 °F 0.0179 25.7 0.0181 26.1 0.0422 60.8
70 °F 0.0178 25.7 0.0180 25.9 0.0182 26.2
The "thermal coefficient" to cool high temperature, high humidity air (i.e., 95.8)
is almost four-fold greater than that to cool or heat dry air (i.e., 25.7 to 25.8).
17: Ventilation Costs 29 printed 09Jul08
5. To calculate cooling cost, one must have both …
• a representative "thermal coefficient" from Table 17.3 and
;
• an ACDD from Appendix C.
But cooling only uses electricity for an energy source. (
Eqn.
17.15a
or
Eqn. 17.15b
with Btu/year cooling from Equation 17.14c.
17: Ventilation Costs 30 printed 09Jul08
Example #17.5: Find the annual cost
to cool 1000 cfm of dry 90°F
make-up air ["A"] to 65°F ["B"]
with a COP of 4.
The appropriate thermal
coefficient in Table 17.3
= 25.8 Btu/ft³/day/°F.
ACDD in the Colorado-Utah
Rocky Mountain region
from Appx. C . 1000 °F days.
Btu/year cooling = Q [cfm] × 25.8 [Btu/ft³/day/°F] × ACDD (using Eqn. 17.14c)
Btu/year cooling = 1000 × 25.8 × 1000 = 26×106 Btu/yr from the air.
= $132/yr (using Eqn. 17.15b)
17: Ventilation Costs 31 printed 09Jul08
Example #17.6: Find the annual cost
to cool 1000 cfm of humid 85°F
make-up air ["C"] to 65°F ["D"]
with a COP of 4.
The appropriate thermal
coefficient in Table 17.3
. 70.1 Btu/ft³/day/°F.
ACDD in Alabama or Georgia
from Appx. C . 2000 °F days.
Notice in Fig. 17.4 that condensation will
occur at this air's "dew point" of ~ 77°F.
Btu/year = Q [cfm] × 70.1 [Btu/ft³/day/°F] × ACDD (using Eqn. 17.14b)
Btu/year = 1000 × 70.1 × 2000 = 140×106 Btu/yr taken from the air.
= $720/yr (using Eqn. 17.15b)
17: Ventilation Costs 32 printed 09Jul08
Two generalizations represented by these two examples, …
• The cost to cool make-up air in a humid 2000 ACDD climate ($720 per
10³cfm/yr) is about five times the cost to cool air in an arid 1000
ACDD climate ($132 per 10³cfm/yr).
• The cost to cool and heat make-up air in a climate with hot, humid
summers and modest winters can exceed the cost to cool and heat
make-up air in a climate with hot, arid summers and cold winters.
6. A common refrigeration term with a historical origin …
A "ton of refrigeration" refers to the cooling rate produced by
melting one ton of ice (at 0°C) in 24 hours, which is equivalent
to 12,000 Btu/hr or 200 Btu/min.
Further trivia: Typical residential air conditioners are capable of about 1 to 5
tons of refrigeration.
17: Ventilation Costs 33 printed 09Jul08
Energy Conservation (see also the Manual Section 7.12)
Applying the Environmental Pollution Paradigm
to thermal energy can save an employer lots of money
(and reduce greenhouse gases).
Prevent the consumption of heat from non-renewable sources.
Minimize the need to heat.
Recycle excess heat (with some caveats).
Treat (not applicable to heat).
Dispose of the excess heat (the default result of typical LEV).
17: Ventilation Costs 34 printed 09Jul08
1. Prevent the consumption of energy from non-renewable sources, e.g. use
solar collectors for heating.
2. Minimize the amount of heat (or cooling) needed …
a. Reduce exhausted air volume (e.g., optimize exhaust flow rates; use
low-volume/high-velocity hoods or controlled variable volume fans).
b. Reduce heating/cooling and ventilation rates in rarely occupied areas.
c. Insulate walls and ceilings (doesn't pertain to exhaust make-up air).
3. Recycle excess heat (or cooling capacity) …
a. Redistribute excess low-temperature heat to cool zones within a facility.
b. Capture and reuse high-temperature heat via a heat exchanger (either
active (e.g., a rotating "heat wheel") or passive (fixed plate)).
c. Recirculate cleaned exhaust air back into a room only if …
1) exposures can stay below a known health threshold limit,
2) a suitable air cleaner is available to remove or reduce the hazard,
3) a continuous air quality or emission monitoring "system" is available,
4) system failure will not cause permanent harm, and
5) recirculation is not prohibited by a local government regulation.
17: Ventilation Costs 35 printed 09Jul08
The learning goals of Chapter 17:
G Know how to use the nominal cost of $100/year per 1000 cfm per "wg
to estimate the fan operating costs for LEV systems.
N Understand the concept of heating and cooling degree days and how
they are used to estimate the make-up air conditioning costs.
N Be familiar with the differences in heating costs among heating sources
and between heating and fan electrical costs.
Be able to use either Eqn. 17.9b (with Table 17.1) or 17.11 (with Table
17.2) to estimate the make-up air heating costs at a given flow rate Q.
Be able to use the "thermal coefficient table" (Table 17.3) to estimate
the cooling costs at a given flow rate Q.
17: Ventilation Costs 36 printed 09Jul08
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