An office building has a two-level enclosed parking garage with a total capacity of 450 cars, a total floor area of 8,300 m^2 (89,290 ft^2), and an average height of 2.75 m (9.0 ft). The total length of time for a typical car operation within the garage is 2 minutes (120 s).

Question

An office building has a two-level enclosed parking garage with a total capacity of 450 cars, a total floor area of 8,300 [latex]m^{2}[/latex] (89,290 [latex]ft^{2}[/latex]), and an average height of 2.75 m (9.0 ft). The total length of time for a typical car operation within the garage is 2 minutes (120 s).

(a) Determine the required ventilation rate for the enclosed parking garage in L/s.[latex]m^{2}[/latex] (or cfm/[latex]ft^{2}[/latex]) and in ACH so that CO levels never exceed 25 ppm. Assume that the number of cars in operation is 40 percent of the total vehicle capacity (a shopping mall facility).

(b) Determine the annual fan energy savings if the ventilation found in (a) is used instead of 1.5 cfm/ [latex]ft^{2}[/latex] recommended by ASHRAE 62-1989. The ventilation system is operated 16 hours/day over 365 days/year. The cost of electricity is $0.07/kWh. Assume the size of the initial ventilation fan is 30 hp (22.9 kW).

Accepted Answer

(a) The procedure for the new design methodology can be applied to estimate the minimum ventilation requirements for the garage defined in this example:

Step 1. Garage data: N = 450 * 0.4 = 180 cars, ER = 11.66 gr/min (average emission rate for a winter day using the data from Table 7.4), T = 120 s, [latex]CO_{max}[/latex] = 25 ppm.
Step 2. Calculate CO generation rate:
Step 3. Determine the ventilation requirement. Using the correlation of Eq. (7.11) for [latex]CO_{max}[/latex] = 25 ppm, the design ventilation rate in L/s.[latex]m^{2}[/latex] can be calculated:

[latex]L/s.m^{2} = C. f. T[/latex] (7.11)

[latex]L/s.m^{2} = 0.692 × 10^{-3} * 56.6 * 120 s = 4.7[/latex]

This ventilation rate corresponds to 0.91 cfm/[latex]ft^{2}[/latex]. In terms of air change per hour, the ventilation rate required for the enclosed parking garage is:

[latex]ACH=\frac {4.7L/m.s^{2}×10^{-3}×3600s/hr}{2.75m}=6.1[/latex]

TABLE 7.4 Summary of U.S. and International Standards for Ventilation Requirements of Enclosed Parking Garages

Ventilation	Ppm	Time (Hrs)
7.6 L/s.[latex]m^{2}[/latex]	9	8	ASHRAE
(1.5 cfm/[latex]ft^{2}[/latex])	35	1	ASHRAE
7.6 L/s.[latex]m^{2}[/latex]	50	8	ICBO
(1.5 cfm/[latex]ft^{2}[/latex])	200	1	ICBO
	35	8	NIOSH/OSHA
	200	ceiling	NIOSH/OSHA
6 ACH			BOCA
6-7 ACH			SBCCI
6 ACH			NFPA
	25	8	ACGIH
	11/3	8	Canada
	25/30	1	Canada
2.7 L/s.[latex]m^{2}[/latex]	30	8	Finland
(0.53 cfm/[latex]ft^{2}[/latex])	75	15 min	Finland
165 L/s.car	200	ceiling	France
(350 cfm/car)	100	20min	France
3.3 L/s.[latex]m^{2}[/latex]			Germany
(0.66 cfm/[latex]ft^{2}[/latex])			Germany
6.35–7.62 L/s.[latex]m^{2}[/latex]			Japan/South Korea
(1.25–1.5 cfm/[latex]ft^{2}[/latex])			Japan/South Korea
	200	0.5	Netherlands
0.91 L/s.[latex]m^{2}[/latex]			Sweden
(0.18 cfm/[latex]ft^{2}[/latex])			Sweden
6–10 ACH	50	8	United Kingdom
	300	15min	United Kingdom

a) Due to the reduced ventilation rate (from 1.5 cfm/[latex]ft^{2}[/latex] to 0.91 cfm/[latex]ft^{2}[/latex]), the fan energy use can be reduced either by installing smaller fans or by reducing the fan speeds (for fans equipped with variable speed drives). In either case, the theoretical reduction in fan energy use can be estimated using the fan laws (in particular, these laws state that fan power consumption is proportional to the cube of the ventilation flow rate):

[latex]ΔkW_{e,fan}=\frac {k W_{exist}}{η_{m}}.(\frac {\dot{m}_{new}}{\dot{m}_{exist}})^{3}.N_{h}[/latex]

Because [latex]N_{h}[/latex] = 16 hrs/day * 365 days/yr = 5,840 hrs/yr and [latex]kW_{exist}[/latex] =22.9 kW with [latex]\dot{m}_{new}/\dot{m}_{exist}[/latex] =0.91/1.5 = 0.61, the annual fan energy savings can be estimated to be (assuming 90 percent motor efficiency):

[latex]ΔkW_{e,fan}=\frac {22.9kW}{0.9}.(0.61)^{3}.5,840hrs/yr=33,728kWh/yr[/latex]

Based on an electricity rate of $0.07/kWh, the cost savings attributed to savings in fan energy use is:

[latex]ΔC_{e, fan} = \$0.07/kWh*33,728 kWh/yr = \$2,361/yr[/latex]

Question 7.1: An office building has a two-level enclosed parking garage w...

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