Chapter 5: Human Comfor & mechanical Systems Fundamentals DEFINITIONS • •
• • • • • • •
• • •
British Thermal Units (BTU): The amount of heat required to raise the temp of 1lbm of water by 1° Coefficient of heat transmission: the overall rate of heat flow through any combination of materials, including air spaces and air layers on the interior and exterior of a building assembly. Reciprocal of the sum of all the resistances in the assembly. Used to determine the size of a heating system for a building U = 1/(R) Conductance: The number of BTUs per hour that pass through 1sqft of homogeneous material of a given thickness when the temp differential is 1°F. Reciprocal of Resistance. Resistance: Number of hours needed for 1BTU to pass through 1sqft of material or assembly of a given thickness when the temp differential is 1°F. Reciprocal of Conductance. Conductivity: Number of BTUs per hour that pass through 1sqft of homogeneous material 1in thick when the temp differential is 1°F. Dew Point: Water vapor in the air becomes saturated and begins to condense to drops of water Dry-bulb temperature: The temperature of the air-water mixture as measured with a standard drybulb thermometer Enthalpy: The total heat in a substance, including latent heat and sensible heat. Latent Heat: heat that causes a change of state of a substance, such as the heat required to change water into steam. How many BTUs to change 1lb of water from freezing to a boil? 212° - 32° = 180° 180° x 1btu = 180btu Sensible Heat: Heat that causes a change in temperature of a substance but not a change of state. Insolation: Total solar radiation on a horizontal surface. Specific Heat: Number of BTUs required to raise the temp of a specific material by 1°f. Latent heat of evaporation = 1000Btu
HUMAN COMOFRT •
Human Comfort is based on the quality of primary environmental factors: Temperature, humidity, air movement, temperature radiation to and from surrounding surfaces, air quality, sound, vibration. Comfort factors: Temperature Humidity Precipitation Radiation Air movement -
•
Human Metabolism • •
•
Not efficient in conversion of food to energy. It must give off excess heat in order to maintain a stable body temp. Body’s heat production is measured in mets: (1.84 BTU/hr-sqft) the energy produced per unit o f surface area per hour by a seated person at rest. 98.6°: no leat loss from conduction, convection or radiation When temp rises above body temp, heat flow reverses and evaporation occurs Adult at rest = 400 BTU/hr Adult w/ moderate activity = 700 – 800 BTU/hr Adult w/ strenuous activities = 2000 BTU/hr People in cooling calculations = qp which is the number of people x BTUs per person
•
•
Body loses heat in three primary ways: Convection: transfer of heat through the movement of a fluid medium, either a gas or liquid. When air temperature surrounding a person is less than the body’s skin temp (around 85°). The body heats surrounding air which rises and is replaced with cool air Evaporation: When moisture changes to a vapor as a person perspires or breaths Radiation: transfer of heat energy through electromagnetic waves from one surface to a colder surface Conduction: transfer of heat through direct contact If body cannot loose heat one way, it must loose it another. If air temp is above the body temp there can be no convection transfer b/c heat always flow from high level to a low level (second law of thermodynamics)
Air Temperature • • • • • • •
•
A measure of stored heat energy Temperature is never transferred, only heat energy Primary determinant of comfort General comfort range: 69°F – 80°F Tolerable range: 60°F – 85°F Dry-bulb temperature is measured with a standard thermometer Wet-Bulb temperature is measured with a sling psychrometer Effective temperature: An index of thermal sensation, not a measure of actual thermometer temp but a measure of a combination of several comfort indexes Dry bulb temp Relative humidity Radiant energy convection Psychrometer: A device that consists of a thermometer with a moist cloth around the bulb. Thermometer is swung rapidly in air, causing moisture in cloth to evaporate. In dry air moisture evaporates rapidly and acquires latent heat, which produces a low wet-bulb temp. Large difference between wet-bulb and dry-bulb indicates relative humidity In moist air, less moisture evaporates from the cloth, so wet-bulb temp is higher
Humidity •
• • •
Relative humidity is the ratio of the percentage of moisture in the air to the maximum amount the air can hold at a given temp without condensing. Comfortable between 30% - 65% Tolerable between 20% - 70% Dew point: temperature that water vapor in the air becomes saturated and condenses into droplets As temp drops air can no longer hold as much water vapor and vapor condenses Important in summer months b/c as temp rises, body can lose less heat through convection and must rely on evaporation However, as humidity rises it is more difficult for perspiration to evaporate, hence one feels hotter.
Air Movement •
Tends to increase evaporation and heat loss through convection This is why one will feel comfort in high temps & humidites if a breeze is present Also explains windchill
Surface Temperature • • • • •
If surface areas of surroundings are colder than skin (85) the body loses heat through radiation If surrounding surfaces are warmer, body gains heat. The rate at which radiation occurs depends on surface temperatures of body and adjacent, the viewed angle and emissivity Viewed angle: angle formed between the measuring position and the outer edges of the object. Emissivity ( ): the ability of a matereial to absorb and then radiate heat.
•
•
• • • •
Thermal conductivity (k): ability of a material to transmit or conduct heat or electricity. Is the amount of heat transmitted in one hour thru one sq ft of a 1” thick material for each degree fahrenheit The ratio of the radiation emitted by a given object or material to that emitted by a blackbody at the same temp. Shiny objects have very low emissivity (shiny foil on insulation to reduce heat transfer) Mean radiant temperature (MRT): A weighted average of the various surface temps in a room and Is the average radiant temperature of surroundings and is independent of air temperature the angle of exposure to the occupant to these surfaces as well as any sunlight present To determine effects of surface temps on comfort, all room surfaces with their temp and positions must be taken into account. As MRT is low, comfort zone shifts towards higher ambient temperatures Operative Temperature: an average of the air temp of a space and the mean radiant temp (MRT). It can be measured with a globe thermometer Globe thermometer: a thermometer inside a black globe which can account for both the air temp and radiant effects from surrounding surfaces Specific heat: ability of a material to store heat in relationship to the materials weight Thermal lag factor: numerical representation of the time it takes radiant heat gain to become absorbed into room and and become part of lad on the cooling system
Clothing • •
Insulator moderating the effects of conduction, convection and radiation. Clo: unit to quantify the effects of clothing on the person, equal to the typ business suit…0.15 Clo/lbm (.80 BTU)
Ventilation • • • •
Required to provide oxygen and remove carbon dioxide, odors and contaminants Building codes specify min. fresh outdoor air that must be circulated (in CFMs) Mech systems are designed to filter and recirculate much of the conditioned air and also too introduce a percentage of outdoor air Where exhausting of air is required building codes specify minimum exhaust rates per square foot of floor area or how often complete air change must be made. Must exhaust directly to outside. NO recirculation Toilets Kitchens Spaces where noxious fumes are present
MEASUREMENT SYSTEMS Comfort Chart • • • • •
Shows the relationships among temperatures, humidity and other comfort factors With tolerable humidity levels between 20 – 70% and preferred levels between 30% - 65%, comfort chart show hat as humidity increases, air temp must decrease to provide same amount of comfort as felt with lower humidity levels. As temp drops below recommended levels, radiation in the form of sunshine or mechanical radiation is needed to maintain comfort The lower the temp, the more radiation is required. As humidity and temperature increase, air movement is required to maintain comfort
Psychrometric Chart • • •
Shows air at different temperatures and humidities Used to graph total of energy stored in air called enthalpy (sensible and latent heat) To cool and simultaneously dehumidify requires removing both forms of stored heat so only need to look at change in enthalpy Psychrometry: the study of water vapor content of air
• • •
Sling psychrometer: instrument that measures relative humidity or wet bulb temperature and is composed of dry and wet bulb thermometers. Difference between two thermometers is compared to determine relative humidity A Graphical representation of the complex interactions between heat, air and moisture A way to calculate the amount of heat and moisture that must be added or removed by the HVAC system. To determine the dew point of the moisture in the air to avoid condensation of interior surfaces and inside the building.
EXTERNAL AND INTERNAL LOADS A building must resist either the loss of heat to the outside during cold weather or gain of heat in hot weather. Any excess heat gain or loss must be compensated for with passive energy conservation or with mechanical heating or cooling • External factors that cause heat loss: air temp and wind • External factors that cause heat gain: air temperature and sunlight. • Internal factors: people, lights and equipment • Heat is transferred between the outside and the inside through conduction, convection and radiation Conduction: transfer of heat through direct contact Convection: transfer of heat through movement of air Radiation: transfer of heat energy through electromagnetic waaves from ne surface to a colder surface
Heat Loss Calculations • • • • • • • •
Heat is lost through the envelope and through infiltration Every material has its own unique property of conductivity Degree Day: temp difference between a 24 hur ext temp and an intreior temp of 65° Design Day: A day hotter or colder than 98% of the rest of the year Conductivity (k): amount of heat lost through 1sqft of a 1” thickness of a material when the temperature differential is 1°F Conductance (C): the same property, but when the material is a thickness other than 1” Resistance (R): is the number of hours needed for 1 BTU to pass through a material of a given thickness when temperature differential is 1°F Conductance and resistance are related by: R = 1/C
•
Values for k, C and R are given in standard reference texts & in the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook of Fundamentals
•
U-value: combined conductance of an assembly Is reciprocol of the sum of resistances When building assembly consists of more than one material, the value used to calculate heat loss is the coefficient of heat transmission (U). However the value of U is not the sum of all conductances if individual materials. Instead must be calculated by : -
U = 1/R •
For an entire area of one type of material, this value is multiplied by the total area to get the total heat loss. This formula is: qc = UAt qc = U(A)(TINSIDE – TOUTSIDE) qc = U(A)24(DD)
• •
In order to calculate the heat loss for an entire room or building, the heat losses of all the different types of assemblies must be determined and added. Example: if summation of the R-values (R) of the assembly is 20.15, then the overall coefficient of transmission is: U = 1/R = 1/20.15 BTU/hr-ft2-°F = 0.05 BTU/hr- ft2-°F
•
•
The value for t is determined by subtracting the outdoor design temperature from the desired indoor temperature in the winter, (70°). Outdoor design temperatures vary with geographical region and are found in the ASHRAE Handbook or are set by local codes Important aspect of heat loss calculations and the use of the psvhchrometric chart is to determine the dew point of the moisture in the air to avoid condensation of interior surfaces and inside the building. Example: air at 70 and 35% relative humidity has a dew point of 41F. Moisture will condense on surfaces at or below this temperature Example 2: if outdoor temp is 0F and indoor temp is 70F somewhere inside the wall assembly the temp is 41F or less. Water vapor from inside the building permeating the construction would condense on this surface, damaging the wood construction and possibly negating the effectiveness of the insulation. vapor barrier must be placed on warm side of the insulation.
Heat Gain/Cooling Load Calculations •
Heat is produced by radiation of the sun on glazing, by building occupants, lighting and equipment. Each factor varies with occupancy: Residence = dominated by gains from envelope & glazing Large office = large no. of occupants, large no. of lights and equipment. Heat loss through bldg envelope calculated similar to heat loss using the overall coefficient of heat transmission and the area of the building assembly (q = UAt). However temp differential is not used directly. Instead a value known as design equivalent temperature difference (DETD) DETD accounts for air temp differences, sun effects, thermal mass storage, colors and daily temp range. Values are published in ASHRAE. -
•
•
• •
•
Heat gain through glazing (qr or SHGF) = multiply area of glazing by the design cooling load factor (DCLF) Values are published in ASHRAE Heat gain from people (qp): number of people x BTU per person Total sensible heat = multiply number of occupants by 225 BTU/hr Occupants produce sensible heat and latent heat in form of moisture from perspiration and breathing. 225 BTU/hr = sensible heat gain from occupants. Heat gains through lighting (qi): = multiply total wattage load of bldg by 3.41. One watt = 3.41 BTU/hr Qi = 3.4(W) Where: W = wattage of equipment Fluorescent & discharge lights w/ elec ballasts = multiply fixtures individual BTU/hr by 1.25 Heat gain from equipment (qm):
-
•
qm = 1500 x Bhp •
•
Where: Bhp = Brake horse power (2545 BTUs) High mass materials – mitigates effects of heat gain from solar radiation & air temp Masonry, concrete and tile slow transmission of heat. Day: absorb heat energy and store it Night: as temp drops below surface of the mass, energy lost to atmosphere instead of into bldg
Cooling load temperature differential: related to conduction and radiation Factors including Mass and storage capacity Color Orientation Total heating load: using all factors -
•
roof)
QTOTAL = qc + qs + qi Conductance (qc) may consist of several “sub” qc’s, one for each surface (wall, floor,
Infiltration • • •
Heat gain through infiltration when outside temps are high. Transfer of air into and out of a bldg thru doors, cracks around windows, flues and vents. Heat gain thru infiltration: qi = V(1.08)t Step 1: determine amount of air infiltration Step 2: determine amount of heating/cooling required to bring air to desired temperature 1.08 BTU-min/ft3 = Specific heat of air (amount of heat that air at a certain density can hold V = calculated via air lost through cracks, doors openings, etc (estimated with use of tables) Air changes: 6 – 10 Required to know number of air changes per hour (Qcfh)
-
•
Qcfh = N x V Where: N = number of air changes V = building volume in ft3
•
Crach method: based on the number of linear feet of crack Example: 3’ x 6’ window = 3’ + 6’ + 3’ + 6’ = 18’ Double hung would have to add 3’ for intermediate Amount of infiltration is determined by a table which considers windspeed and window type Value is then multiplied: Qcfh = LF x CFH/lin.ft. -
•
Finally, the amount of heating or cooling required may be calculated by:
qi = .018(Qcfh) )t = .018(Qcfh) (TINSIDE – TOUTSIDE) Temperature gradients: temperature change across a single material tlayer = (RLAYER/RTOTAL)
CLIMATIC TYPES AND DESIGN RESPONSES •
Mechanical systems w/o regard to passive design strategies appropriate to local climate: Increases cost Wastes energy Contributes to pollution Ignores desirable regional characteristics of architecture USA = four basic climate types/zones Cool: Canada, Northern middle USA & Mountainous regions of WY & CO Temperate: middle latitudes, NW & NE Hot-humid: SE Hot-arid: SoCal to SW Texas -
•
•
Cold climates: Minimize surface area to reduce heat loss Cubical shape Partial underground Minimize Northern exposure Minimize windows & doors Landscaping & bldg design should block winter winds Due to lack of direct sunlight in winter, passive solar not appropriate Mechanical heating & active solar heating required Temperate climates: significant heatloss in winter Minimize Northern exposure Block winter winds Solar heat gain in winter desirable bldg length should be oriented East and West to max Southern exposure. Summer, same south face = deciduous trees for shading & mechanical devised (awnings) To mitigate effects of daytime heating, provide nighttime venting Solar heating – active and passive work well Hot-humid climates: most difficult to design for w/o mechanical cooling Plan form max natural ventilation Narrow floor plans w/ cross ventilation Large open windows, porches breezeways Shade with vegetation Double roof Thermally light weight so as to not store daytime heating & release at night Hot-Arid climates: Shading from direct sunlight Diurnal effect – wide variations bet day and night temp Materials with high thermal mass Evaporative cooling with pools Roof ponds provide evaporative cooling & high thermal mass -
•
•
•
