Physiological Tolerance

Human societies have existed centuries without modern technology to provide comfort. Outdoor air dry-bulb temperature conditions range from -60^oF to +130 ^oF. Heating consisted of open fires and there were no means for cooling except hand-held fans. Humans cannot survive at -60^oF together with high wind velocities (clothing cannot provide enough protection and no part of the body can be exposed) and at +130^oF together with high humidity (body can not reject heat by heat transfer or perspiration).

Normal human body temp is 98.6^oF ( 37^oC). Anything outside the temp range of 98 oF to 99^oF is not normal. Humans can survive for a very short interval when their body temperatures are 60^oF (hyperthermia) and 110^oF (fever). The human body produces heat internally through metabolism. The fuel source is food (solid and liquid). Body heat must be rejected to the surroundings or else the human body cannot survive. The metabolic rate (body heat production) depends an activity.

	Sensible	Latent	Total
Resting / Sleeping	200	100	300
Moderate office work	250	200	450
Heavy factory work	600	900	1500
Bowling, light exercise	1500	1500	3000
Vigorous exercise	2000	2500	4500

Winter

Below 30^oF and no clothing protection over a prolonged period of time can cause hypothermia. However, vigorous exercise can balance out the hear losses through light clothing. Body heat can be preserved within proper clothing even against high winds. Skin exposure to temps of - 30^oF and below for a few minutes can cause frost bite. There is very little moisture in air below 30^oF (most of it has condensed and turned to ice), Below 0^oF the moisture content is almost zero. If outdoor air DB is say 0oF and the indoor space is maintained at 75^oF and ventilated with outdoor air and there is also outdoor air infiltration, then the relative humidity levels inside can drop to below 10% if there is no humidification. This can affect people with breathing problems and there is static electricity sparks associated with metal surfaces and silk clothes.

Summer

Heat produced by a person must be absorbed by surroundings. Below 75^oF and 50% RH (standard indoor comfort conditions), the heat transfer is mainly sensible. 70^oF to 90^oF, the heat transfer can be sensible and or latent depending on the temp and RH and also level of physical activity. Above 100^oF and low RH, the heat transfer is latent through sweating or perspiration since the body temperature is 98.6^oF. Above 100^oF and high RH, not enough latent heat transfer can occur through perspiration and the body will suffer heat exhaustion. Controlling humidity is therefore a major factor in comfort air conditioning and a major cost. Moisture must be added in winter and removed in summer.

Judgment must be applied in determining comfort conditions. For example a person exercising in a gymnasium is trying to get hot and perspire so blasting him with cold air to bring his temperature down instantly can defeat the purpose of comfort air conditioning. Conditions in a Sauna (steam bath) can not be 75 degree F, 50% RH since the occupants are trying to lose weight through perspiration.

So different comfort environmental conditions apply to different space (room) applications. Indoor air comfort conditions for a particular type of space for natives of a hot topical climate (say Singapore) are not going to be the same as those for natives living in cold climates (say Norway). ASHRAE Standard 55-1992 comfort standards for different types of spaces (say 75^oF DB and 50% RH for hospitals in summer and 72^oF DB and 30% RH in winter) try to cover the USA which has a wide range of climates. Local hospital codes in Florida and Minnesota make the adjustment.

Comfort is also subjective feeling that varies with each individual based on health mood, activity, age, etc. There is no "comfort meter". Comfort air conditioning is an approximate science. It becomes a precise science to building design architects and engineers, when codes and standards dictate what the indoor conditions must be. A small + or – tolerance range is usually included in the standard. Indoor air conditions in different types of hospital spaces (operating rooms, patient rooms) and pharmaceutical laboratories can vary considerably and must be maintained.

HVAC definition of comfort

        Thermal comfort exists when a person is surrounded by an environment (air) in terms of temperature, humidity, and air motion which allows the person to lose the heat generated by metabolism at the same rate that it is being produced without a conscious effort on the person's part to lose this heat. Air conditioning comfort also requires adequate ventilation (maintenance of oxygen content of air) and good air quality (clean air that is free of pollution).

This HVAC definition of comfort is supposed to cover a very wide variety of situations. Judgment must be applied in determining comfort conditions for various types of people performing different activities. A person at rest generates less than 400 Btu/hr and a person working out in a gymnasium generates 2000 Btu/hr to 4000 Btu/hr (the range is 400 too 4000).

HVAC comfort definition does not apply to certain situations such as gymnasiums. The person wants to perspire and get hot. The rate of heat removal is not supposed to equal the rate of heat generation. It is not possible to apply the definition when more than one person is involved. No two persons have the same rate of heat generation at any given time.

In today's modern affluent society in the U.S., the HVAC definition of comfort is taken for granted. It is expected at the work place. The first cost of HVAC in a modern 100 million dollar commercial office building can be about 20 percent ( 20 million dollars). The annual energy cost is mainly due to comfort heating and air conditioning. Power blackouts in summer are mainly due to summer air conditioning of commercial buildings. In winter there are other fuel alternatives such as fuel oil and natural gas and even coal and wood fireplaces.

The definition of comfort in architectural and engineering design offices is "whatever is stated in the standard or code". You don't worry about the needs of individuals, by age health, sex, etc.

Mean Radiant Temp (MRT)

        Radiant Heat energy is part of the electromagnetic wave spectrum; Radiant energy from the sun passes through space and atmospheric air surrounding the earth and strikes the solid objects on the earth. Radiant heat energy from the sun has no impact on the 93 million miles of space that it passes through, and it has little impact on the 10 miles of air that it passes through. The air is not heated significantly and the temp of the air does not increase significantly as radiant heat energy from the sun passes through the air to the earth’s surface. The radiant heat energy is heats the earth’s surface then transfers the heat by conduction and convection to the air. So the air temperature drops with increasing elevation.

        Radiant heat is transferred from a hot object to a colder object without affecting (heating) the space and air in between. A person's body temp is between 98^oF and 99^oF. If the air conditioned room temp is about 75^oF, then the person's external skin temp will average about 85^oF. All hot objects above 85^oF will transfer heat by radiation to the person without heating up the air in between which will remain at 75^oF.

        Solar radiation from the sun will pass through the window (glass) and it will heat the person. It also heats all the objects and surfaces in the space which then heats the air in the space through convection and conduction. The amount of solar radiation heat entering through glass can be as much as 300 Btu per hour per square foot. The impact of the radiant heat in winter can be comfortable but in summer it can feel uncomfortable. The direct transfer of radiant heat to the objects in the room can be blocked by drawing (closing) the shades or blinds.

        In summer the solar radiation falling on the outside surface of walls and roofs (it can not penetrate or pass through solids very easily) will heat the wall and roof surface by heat absorption. If the outside surface is black or coarse, the heat absorbed can be considerable. Depending on the intensity of the solar radiation, the surface DB temperature can reach about 200^oF.

        If the wall or roof is not insulated and the thickness is small (say 1/2 inch thick aluminum ) then the inside surface temp of the wall or roof can be very high and close to the outside surface of say 200^oF temp. Air conditioning will still maintain the space air temperature at 75^oF. In winter the opposite happens and the inside surface temperature of 1/8 inch single glass can be close to the outside air temperature of say -40^oF although the space air temperature is 75^oF. The high inside surface temps of the envelope will transfer radiant heat to the objects (people) in the room without heating the air in the room. In winter people are going to lose heat through radiant heat transfer to the cold envelope inside surfaces. The inside air temperature will still be 75^oF.

        So it is possible to have the air in the room at 75 degree F (comfortable) and the relative humidity at 40 % (comfortable) and the air motion at 300 feet per minute (comfortable) and the people in the room can be very uncomfortable because of radiant heat gain and loss.

        In winter the people in the room will transfer heat to the walls, glass and roof. In other words they lose heat by radiation and feel colder, although the air in the room may be kept warm at 80^oF. The opposite will happen in summer the walls, roofs are glass that are at high temps will heat the people in the room by radiant heat transfer although the air in the room might be at 70^oF.

        Radiant heat can have a significant impact on comfort level of people in the room. Maintaining ideal DB temps, relative humidity and air motion may not be enough to produce comfort if the radian heat transfer component is not controlled. One of the objectives of comfort air conditioning is to maintain the inside surface temps of walls and roofs, as close as possible to room temp. This is done by providing adequate insulation, by maintaining very low U-values for the building envelope (wall, roofs, windows, skylights and doors).

Mean Radiant Temperature accounts for the temperature impact of surfaces according to the angle of influence. This is the conical angle made by the person (usually the head which is the only exposed surface of the body) and the perimeter of the surface. The person closer to the surface will make a larger angle than a person farther away and will therefore be affected with a higher radiant heat transfer with the surface.

Figure ?

In the figure above, the east surface is not in direct sight of the body. It is blocked by the screen. So there is no radiant heat transfer from the glass surface temperatures in summer and winter. Mean Radiant Temp (MRT) is the weighted average temps of all surfaces in direct sight of the body. It tends to stabilize near the room temp. Note if the hot (or cold) object is not in direct sight of the body then its temp is not included in the weighted averages.

MEAN RADIANT TEMP (MRT)

Figure ?

Operative Temp is the uniform temp of a radiantly black enclosure in which the occupant exchanges the same amount of heat by radiation and convection as in the actual non-uniform environment. Operative temp can be expressed numerically and approximately as the average of the mean radiant temp and the room air temp.

Direct (thermal) Indices of Comfort

1. Dry Bulb Temp
2. Dew Point Temp
   Wet Bulb Temp
   Relative Humidity
   Humidity Ratio
3. Air Movement

Derived / Calculated (thermal) Indices of Comfort

Mean Radiant Temp
Operative Temp ( average of MRT and DB-T)
Humid Operative Temp
Heat Stress Index
Index of Skin Wetness (skin surface evaporation index)

Heat Exchange between the body and the surroundings is influenced by:

Dry Bulb Temp
Relative Humidity (or WB, DP, W)
Thermal Radiation
Air Movement
Amount of Clothing (Clothing Factor)
Activity Level
Direct Contact with surface not at the body temp.

Body Heat Balance Equation

Rate (per unit time) of body heat generation = Rate of body heat loss

If there is no heat balance over a prolonged period, then there will be hypothermia (cold) or heat exhaustion (hot)

M = metabolic ( human) body heat generation rate ( = 0 means the body is dead)
E = Heat loss by evaporation (perspiration)
R = Radiant heat loss (+ve) MRT < body temp     Or gain (-ve) MRT > body temp
Cd = Condution heat loss (+ve) or gain (-ve)         CV = Convection heat loss (+ve) or gains (-ve)
S = body heat storage rate (heat not dissipated to the surroundings can be stored up to a point)

Body heat balance can be achieved by controlling the surrounding space for:

1. DB temp
2. RH
3. Air Motion
4. MRT

Indoor Air Contaminants

This can seriously affect the comfort conditions of a space. Toxic gases viruses, bacteria, etc can affect the health of occupants. Dilution of the room air through ventilation is not usually satisfactory. For example air dilution with high ventilation rates for smoke from fires and carbon monoxide from automobiles (underground parking) is not effective. The smoke and carbon monoxide must be removed at the generating source and exhausted.

Minimum Air Motion

Low air velocity affects the ability to maintain uniformly comfortable air conditions throughout the room. Occupant comfort is affected by low or no air motion although the temp and relative humidity is within the comfort zone. For the .....it is normal to supply a room with a minimum of 0.6 to 0.8 cfm/ft2 of floor space. This might be inadequate if the room ceiling height is high (over 10 ft). Codes and standards for health care facilities (hospitals) specify air supply rates as air changes per hours (ACH). The minimum ACH per a typical office is about 5 to 6 ACH with 8 being more comfortable. Distribution of air supply diffusers can effect comfort.

ASHRAE Comfort Chart

The chart shows a comfort region that extends between 68 DB and 82 DB and relative humidity up to 60%. For energy and economic reasons, winter indoor design conditions would be closer to 68 DB, 30% RH and summer indoor design conditions would be closer to 82 DP, 60% RH. Actual indoor design conditions for different types of rooms and applications should be based on codes and standard (EX. ASHRAE Standard 55 for indoor thermal environment comfort)

Zoning Considerations

Maintaining indoor thermal comfort conditions depends on thermal space zoning, temperature and humidity control systems, indoor air distribution and circulation, magnitude of fluctuating loads and the HVAC systems sensitivity and capacity to maintain design indoor conditions during all operating hours. Whenever possible, rooms that require the same environmental conditions should be located near each other and thermally zoned with the same control system zoning must consider tenant zones besides thermal zones. This would increase the number of zones, the number of control systems, and the cost of the HVAC systems.

Thermal Zones (5 Zones) Thermal Zones (9 Zones)Tenant Zoning

Figure ?

Ventilation & Exhaust

The term ventilation refers to outdoor air introduced into a conditioned space. It is not the total amount of air supplied to the space. The total amount of air supplied to a space is determined by the cooling requirements of that space. The total supply air includes the minimum ventilation air specified for the space by the applicable code.

The purpose of ventilation is to introduce contaminant free air into the space and to replenish the oxygen content of the air in the space. The purpose of exhaust is to remove contamination in the air at the rate or which it is introduced. The ventilation air introduced can not be less than the exhaust air quantity since it is the make up air quantity for the air exhausted. The ventilation air can exceed the exhaust air depending on the code requirements for outdoor air for the space (usually specified on CFM per person, air changes or CFM per unit area).

People factors

Are subjective and can not be controlled by architects and engineers. The comfort requirements of people can vary considerably depending on these factors.

Age
Sex / gender (male or female)
Health
Body type (thin, fat)
Genetic / ethnic (people from different climates, complexion, dark skin vs. white skin).
Mood / Attitude
Incentive / Drive / Enthusiasm
Body rhythm cycle
Diet (before or after heavy meal)
Activity (office, work, exercise workout)
Sensory organs (eyesight, hearing touch, etc)
Clothing / Exposure
Social situation (relaxed, in big trouble, panic, fear)
Relative living standards (it takes more to make a rich lazy slob comfortable)

Space factors

Space factors are designed by architects, interior designers, lighting consultants, acoustic consultants and engineers and can be controlled by them. It is their responsibility.

HVAC

Temperature (heat intensity)
Enthalpy (heat quantity in the air)
Humidity (relative, dew point).
Pressure (atmospheric, building pressurization)
Air Motion (mechanical, fans, diffusers)
Ventilation (air freshness, oxygen content)
Air Quality (pollution, contaminant, odors)

Architectural

Occupancy (area sqft per person, volume cuft per person
Space configuration ( ceiling height, area/person, area/volume per occupant).
Space enclosure (proximity of space volume surfaces to the occupants (working in a small amount of space versus a large office.
Services
Lighting
Acoustics
Aesthetics
Interior design

Design Criteria

People, space and economic factors determine design criteria.

Figure ?