ME 414/514 HVAC Sys. – Topic 4 – Indoor Air Quality (IAQ) & Thermal Comfort Indoor Air Quality and Thermal Comfort
The indoor environment of a building may be comfortable, but unhealthy (‘Sick Building Syndrome’). Both aspects of IAQ must be considered in the design of an HVAC system. The following three publications provide detailed guidelines for comfortable and healthy environments:
ASHRAE Standard 62 ASHRAE Standard 55
ASHRAE Handbook of Fundamentals
IAQ Contamination Control
Source elimination/modification – the most effective solution Outdoor air, ventilation – one of the most common methods.
Air Distribution – requires creation of pressure differentials in the room to control the flow of contaminants.
Air cleaning – another common method, often used in conjunction with outdoor ventilation.
Gas removal by chemical means (adsorption, etc.) Particulate removal by mechanical means (filters).
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Contamination (Health Concerns)
1. Carbon dioxide, CO2 2. Carbon monoxide, CO 3. Sulfur oxides, SO2, SO3
4. Nitrogen oxides, NO, NO2, N2O 5. Radon
6. Volatile organic compounds (VOCs) 7. Particulates
Control
1. Source elimination/modification 2. Ventilation (outdoor air)
3. Air distribution 4. Air cleaning
The text provides an example calculation using the contaminate balance in Example 3.7 and we will work problems in class.
For an example of the effective use of outdoor air, refer to Figure 3.16 in the text. 𝐸𝐸𝑣𝑣 = 𝑉𝑉̇𝑜𝑜− 𝑉𝑉̇𝑉𝑉̇𝑜𝑜𝑜𝑜𝑜𝑜= 1−𝑅𝑅𝑆𝑆1−𝑆𝑆
where
Ev = effectiveness of outdoor air use
𝑉𝑉̇𝑜𝑜𝑜𝑜= rate at which unused outside air is exhausted
S = stratification factor, the fraction of supply air bypassing the occupied zone R = recirculation factor, the fraction of return air that is recirculated
Filter Sizing
We would like high efficiency filters, fairly high dust handling capacity, but also low resistance. High resistance values increase the fan size, thus increasing operating cost. The figure below shows a schematic and the nomenclature used in filter selection.
Equations 3.30 and 3.31 have the outdoor air requirements for a certain configuration. Example 3.11 shows how these equations are used in filter selection.
Filters (Particulate Removal)
Efficiency – a measure of the ability of the media to remove particulate matter from the air stream.
Air flow resistance – the pressure drop across the filter.
Dust holding capacity – the amount of particulate that can be held by the filter at a specified flow rate before the efficiency drops dramatically. Figure 3.17 shows typical performance data for air filters.
Thermal Comfort
The human body controls its temperature by:
• Metabolism – the rate at which chemical energy is converted to thermal energy (Table 3.1, assuming an average body surface area of 19.6 ft2).
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• Blood circulation – controls the rate at which thermal energy is carried to the skin for dissipation.
• Respiration – air (and CO2) is expelled at the body temperature and in a saturated state.
• Sweating – a method to increase energy transfer from the body when blood circulation is insufficient.
Engineers like to quantify things.
• Activity level – Table 3.1 , 1 met = 18.4 BTU/(hr-ft2) • Clothing – insulating value, 1 clo = 0.88 (F ft2 hr)/BTU • Dry bulb temperature – easily measured
• Humidity – can be quantified using a psychrometric chart • Air velocity – can be defined using fluid mechanics
• Thermal radiation – mean radiation temperature, globe temperatures, operative temperature, wet bulb globe temperature, humid operative temperature, …
Thermal radiation
Cold and warm surfaces in the occupied space may cause a person to feel uncomfortable, even though the conditioned air may be in the comfort range. The reason for the
discomfort is due to thermal radiation between the body and these surfaces.
The mean radiant temperature (Tmrt) represents the average temperature of the surfaces in
the space that would simulate the real environment. This temperature can be determined using a globe thermometer, a dry bulb thermometer, and an anemometer to determine the average air velocity in the room.
The mean radiant temperature (in R or K) can be estimated from:
(
g a)
g
mrt T C V T T
T4 = 4 + −
Ta = dry bulb temperature (R or K) Tg = globe temperature (R or K)
C = mystery constant (0.103 x 109 (IP units) or 0.247 x 109 (SI units)) V = average air velocity (ft/min or m/s)
The book discussed radiation exchange between surfaces at different temperatures (Equation 3.8):
∑
− ≈ n n n cl r F T TThe operative temperature is rad con mrt rad a con op h h T h T h T + + =
Where hcon is the convective heat transfer coefficient and Ta is the dry bulb temperature.
Tmrt is the mean radiant temperature as defined on the previous page and on page 70 of
the text. There are problems with this derivation, as the effective radiative heat transfer coefficient, hrad, is not clearly defined. This arises from linearization of the
Stefan-Boltzman Law for thermal radiation heat transfer. You’ll find a description of the methodology in your undergraduate heat transfer book. However, this detail does not seem to matter (see next paragraph).
The Operative Temperature (Top), is the average of the mean radiant temperature and the
ambient temperature, weighted by their respective heat transfer coefficients. For most applications, the text says that the Operative Temperature can be simply taken as the average value (without weighting). The Operative Temperature is required to determine comfort conditions from the comfort chart (Figure 3.5).
Adjustments for conditions other than specified in the comfort chart can be determined using Figures 3.6 through 3.9 as described in section 3.3.2.
