SYSTEMS - MECHANICAL Flashcards
3 WAYS to transfer heat
Convection - transfer of heat through movement of gas or liquid. Only when air temperature surrounding person is less than the body skin temperature, around 85°F. body heat air, hot air rises, replaced by cooler air
Radiation - heat transfer through electromagnetic waves from one surface to colder surface.
Conduction - transfer of heat through direct contact between two objects of different temperatures
General comfortable temperature
between 69 and 80°F, tolerable from 60 to 85°F depending on the relative humidity
Emissivity
Emissivity - of an object is a measure of its ability to absorb and then radiate heat. Shiny objects have low emissivity
Emittance
Emittance - of an object is ratio of the radiation emitted by object or material to that emitted by a black body at the same temperature. Shiny objects have low emissivity
Mean Radiation temperature MRT
weighted average of the various surface temperatures in a room and the angle of exposure of occupant to the surfaces, as well as any sunlight present. all room surfaces and the temperatures and positions must be taken into account.
Thermal conductivity K
Thermal conductivity K
Rate At Which Heat Passes Through 1 SF Of 1 Inch Thickness Of Material When Temperature Differential Is 1°F
Conductance C
Rate At Which Heat Passes Through 1 sf Of Thickness Of Material Other Than One Inch When Temperature Differential Is 1°F
R-value Formula
Measure of resistance to heat flow through a given thickness of material. So the higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties
R = 1 / C
Overall coefficient of heat transmission U Formula
U = 1 / sum R
CONDUCTANCE OF THE WHOLE ASSEMBLY
ΔT
- determined by subtracting outdoor design temperature from desired indoor temperature, usually 70°F
Heat Infiltration Formula
transfer of air into building through open doors, cracks, around windows, flues, vents etc
Air Infiltration Heat Loss = Room Volume * ΔT * Air Changes / hr * 0.O18
ΔT=Design Temperature Difference
Design cooling load factor DCLF - formula
Area Of Glazing * DCLF = Heat Gain through Glazing
DCLF - Design cooling load factor
Heat gain through lighting
1 W equals 3.41 Btu/ hr
(for fluorescent lighting W of ballast must be included)
BTU
heat required to raise the temperature of 1 LBM water by 1°F
Enthalpy
total heat of substance, including latent heat and sensible heat
Specific heat
Number of BTUs required to raise the temperature of specific material by 1°F
Minimize First Cost:
Single Duct Constant Air Volume (CAV) or through wall packaged terminal units
Minimize Operating Costs:
VAV, Single Duct CAV, Hydronic Convectors, Closed Loop heat pump
Control Air Quality/Velocity
VAV (all types), Single Duct Constant Air Volume (CAV), Multizone
Individual Control:
VAV, Constant Air Volume (CAV) Reheat, Multizone, Air-water induction, fan-coil terminals, through wall packaged terminal units
Minimize System Noise:
All Air Systems (except induction) and Hydronic Convectors
Minimize Visual Impact:
Any all-air system
Min floor space or floor height:
Through wall packaged terminal units, induction systems, and hydronic convectors
Minimize maintenance:
single duct Constant Air Volume (CAV), and hydronic convectors
Avoid Chimney
Electric boilers, through wall packaged terminal units, closed-loop heat pumps
Min Construction:
through wall packaged terminal units
Balance point temperature:
temp at which the building does not require mechanical heating or cooling
sensible heat
the temperature of the air
latent heat
the moisture content of the air
Temperature at a point in a wall - Formula
Toutside + [(Rvalue outside of point in wall/Rvalue total) x ΔT]
The average R-value of sloped insulation
The average R-value of sloped insulation is the average of the thickest and thinnest values.
Fibrous filters for dust - maintenance
Fibrous filters for dust, must be replaced
electrostatic filters for dust - maintenance
electrostatic filters for dust, must be wiped down
activated charcoal filters
activated charcoal filters for odors or chemicals
prallel / opposed blade dampers best for
parallel best for on off,
opposed blade dampers based for throttling airflow
ASHRAE standards: 90.1
energy and lighting
ASHRAE standards: 62.1
ventilation
ASHRAE standards: 55
thermal comfort
1 horsepower?
1 ton of cooling ?
1 horsepower = 2544 BTU/hr,
1 ton of cooling = 12,000 BTU/hr
Solar Heat Gain Coefficient (SHGC)
ratio of solar heat gain through a fenestration to the total solar radiation incident (falling upon/striking) on the glazing (0.0 - 0.87)
Shading Coefficient (SC)
ratio of solar heat gain through a glazing product to the solar heat gain through an unshaded 1/8” thick clear double strength glass under the same set of conditions (0.0 - 1.0)
SHGC is considered more accurate.
The Seven Big and Nasty Culprits of Poor Indoor Air Quality
- Combustibles: carbon monoxide (odorless, tasteless)
- Standing moisture: mold (green and black is deadly)
• Warm moisture: legionella (standing warm water, found inside anything)
• Volatile Organic Compounds (VOC): toxic gasses (pain, stain, adhesives)
• Formaldehydes: toxic off gassing (sheetrock)
• Particle board: toxic off gassing (furniture)
• Poisons: toxic gasses, liquids, and solids (can be avoided by not brining into the building as cleaning products)
• Poor Indoor Air Quality gets upgraded to Sick Building Syndrome when:
10% of occupants are sick
20% of occupants complain
Vermiculite
Natural occurring minerals composed of shiny flakes
- When heated they expand to 8 - 30 times original size
- Used for insulation in attics and walls, and is fire resistant
- Sold in the United States between 1919 - 1990, most of it came from a mine in Montana which also had an asbestos deposit…so vermiculite was contaminated
• Asbestos:
- Asbestos:
- A mineral fiber that occurs in rock and soil
Found in insulation (with vermiculite), vinyl floor tile and backing, roofing, pipes, furnaces, etc.
- Released in the air when disturbed (e.g.: during demolition)
- Check for it in all pre 1975 buildings
Radon
- Radon:
- A colorless, odorless, tasteless gas that naturally occurs in soil and water
- Lung Cancer is a concern for those who are exposed to high levels for a high period of time
- Greatest exposure risk is in room that are below grade, or those that are directly in contact with the ground
- The method to reduce radon primarily used is a vent pipe system, which pulls radon from beneath the house and vents it to the outside.
• Polychlorinated Biphenyls (PCBs):
- Polychlorinated Biphenyls (PCBs):
- Manmade material ranging from thin light color liquid to yellow/black waxy solid
- Manufactured between 1929 - 1979 (banned)
- Found in electrical equipment, transformers, fluorescent light ballasts, caulking, plastics, oil based paint, adhesives
- Don’t readily breakdown and can spend long times cycling between air, water, soil
calculate daylighting:
- Measure from bottom of floor to the top of the window (doesn’t matter how big it is)
- You can go 2.5x that length into the building for lighting penetration
- Daylighting Factor
max = 0.2 (window area/floor area)
min = 0.1 (window area/floor area)
• Remember…it should be between 1% - 5%!
• High Efficiency Particulate Arrestance (HEPA) filter
• High Efficiency Particulate Arrestance (HEPA) filter:
the highest efficiency option, typically found in special air cleanser for unusually polluted or IAQ demanding environments like hospitals
Electric heating
- Radiant heat is run through panels or wires to rooms
- Low initial cost , Simple system , Can turn on only in occupied room
- Expensive life cycle cost , Wasteful
- Baseboard heat uses convection to heat spaces
Forced Air Systems - Single Duct
- A single supply duct runs to all rooms with a constant air flow • Rate of air flow is controlled by a damper at each diffuser • Controlled by one thermostat
- Lower cost • Less ductwork • Returns can be ducted or open in the space between the ceiling and floor/ roof above, called a plenum • Easy to operate • Good for controlling IAQ
- Can only heat or cool • Only works when loads are similar throughout a building • Bad for perimeter zones in cold climates • Thick distribution trees • Can be noisy
- Typical residential system • Common in buildings with large open spaces, few windows, uniform loads, like theaters, department stores, exhibition halls
Forced Air Systems - Double Duct aka “Dual Duct” aka “High Velocity
Double Duct aka “Dual Duct” aka “High Velocity” • Combination of two single duct systems, one for hot air, and one for cold air • Two streams are joined at a mixing box controlled by a thermostat in the zone
- Can heat and cool at the same time • Constant airflow volume • Good for perimeter zones • Easy to install • Good for linear buildings
- Twice as much ductwork (one to heat, one to cool) • Both boiler and chiller have to run all the time • The most energy is consumed with this system large fans) • noisy distribution • Hot and cold air produced
- Each room has a thermostat which mixes air in box before entering room • Common in hospitals • Mostly replaced by VAV systems
Air-Hydronic Systems - Fan Coil
Combination Hydronic Four Pipe system and constant air volume that can heat and cool at the same time
- A boiler and chiller each attached to a two-pipe system AND ductwork for the supply air • One of the most efficient systems
- Versatile because it can provide heating and cooling simultaneously
- High initial cost • Highest installation cost because there’s ductwork and plumbing involved
• Sends clean conditioned air through a single duct • A fan blows air over a hot or cool coil in each room • Can be just used for ventilation without heating/ cooling activated
Forced Air Systems - Variable Air Volume
• Air is heated or cooled at a central location and distributed through a single duct. • Thermostat controls a damper at each zone to control the volume of conditioned air into that space.
• Can heat and cool different zones at the same time • Most common and efficient system • Saves energy because it doesn’t have to run peak all the time
- Can’t heat and cool different rooms in the same zone at the same time • A maintenance nightmare! • Requires a lot of interstitial space
- Can be single or multiple single duct systems • A zone can be one or many rooms • System is set to handle hottest or coldest room and rest adjust • Used in large buildings where temp regulation is required
Forced Air Systems - Unitary
- A self contained unit where air comes directly in from the outside, conditioned and sent into the space • Use when ducts are impracticable to run
- Each unit can have it’s own utility bill • One unit is required for each zone • Can run on just electric, but can also connect to hot/cold piping • They’re the units you see in big box stores
Forced Air Systems - Reheat (Constant Volume)
- Return air and fresh outside air are combined and cooled and dehumidified • Distributed in constant volume at a low temperature
- Humidity and temperature can be controlled • Ducts are smaller • Fan horsepower is lower
- Uses more energy because primary air volume needs to be cooled most of the time and reheated
- Terminal: equip. located near conditioned space • Zone: coils are located in ducts to serve an entire zone
• Economizer Cycle: outside air can be used when temps are low enough
Forced Air Systems - Induction
• cabinet, historically installed at floor level in front of windows, or above suspended ceiling,
includes heating/cooling coil, air filter and condensate drain pan. Air is provided to the induction unit from a central air handling unit, along with water from the boiler and or chiller
. The primary air from AHU is used to induce a secondary flow of room air across the coil.
(Unlike the induction unit, the fan coil unit circle it’s room air but the action of a fan)
• Perimeter zoned areas: schools, offices, labs
Hydronic Systems - Hydronic Single Pipe
- single supply and return pipe • hot water is circulated through each register and back to the pipe
- Low initial cost • Simple
- Can’t go very far because water temp drops • Can only heat or cool at one time
- first register will be hot, the next a little cooler, etc • Can be combined with a forced air system, or stand alone
Hydronic Systems - Hydronic Two Pipe
- Like a Single Pipe System, but separate supply and return pies are used
- Doesn’t put used water (that’s cooler) into the supply line for the next register
- Can only heat or cool at one time
- Can be combined with forced air system, or stand alone
Switch over between heating and cooling at predetermined time, slow process, often problem for users.
Hydronic Systems - Hydronic Three Pipe
- Like a 2 Pipe system, but both hot and cold water are mixed in a common return pipe
- Can heat and cool at the same time
- Mixes cold and warm water in return pipe
- Less efficient than a two/four pipe system
- Can be combined with forced air system, or stand alone
Hydronic Systems - Hydronic Four Pipe
Like (2) two pipe systems, but there’s one return for hot and one for cold
- Can heat and cool at the same time
- More expensive due to piping
- Can be combined with forced air system, or stand alone
What is an RTU
A packaged AHU for larger buildings is also termed a Rooftop Unit (RTU).
An RTU is used as a primary system in buildings because it has enough power to regulate the heating and cooling for a building by itself, or with the assistance of an additional rooftop unit.
issues due to having a well-constructed air-tight building
The gas fireplace is not working well,
the doors and windows can be difficult to open,
and odors tend to linger.
The goal is to keep the air tight construction while bringing in fresh air.
Q. Which of the following best describes why it is important for ductwork to follow a simple, direct path?
Ducts are expensive to install.
Ductwork takes up a significant amount of space.
Round ducts maintain better air pressure.
Ducts can pose acoustical challenges.
The correct answer is B.
The correct answer is that ductwork takes up a significant amount of space, and a simple, direct path will help to reduce the amount of space necessary. They are not expensive to install, and pose no acoustical challenges. Although round ducts do maintain better air pressure, that is not a reason to ensure they follow a simple, direct path.
R value formula
Outdoor air, infiltarion formula
qinfiltration = (Aexposed) (infiltration factor)
Aexposed- Area of exposed wall surface, including fenestration.
Infiltration Factor- Should be presented in a tabulated format.
Outdoor air includes mechanically introduced outdoor air.
Cooling Degree Day - CDD
A cooling degree day (CDD) is a measurement designed to quantify the demand for energy needed to cool
a building
CDD is the number of degrees that a day’s average temperature is above 65° Fahrenheit.
Important Formula for Heating Capacity (Btu/h)
Calculating Btu Cooling
1 ton cooling capacity
1 ton cooling capacity
= 12,000Btu/h or 3,516 W
Effective Temperature:
Effective Temperature: fictitious temperature that produces the same physiological
effect as the combined effects of temperature, humidity, and air movement NOT AN
ACUTAL TEMPERATURE.
measuring instruments temp
Barometer: an instrument for measuring atmospheric pressure
Globe Thermometer: used to measure radiant temperature. It’s a dry bulb thermometer, encased in a matte black copper sphere
Hygrometer: instrument used to measure the relative humidity of the air
CFM formula
cfm=(space volume) x ACH / (60 min/hr)
Start with the total volume of air (in cubic feet), divide by the exchange rate (ach/ how quickly you want to replace the air), and the result is the total CFM (cubic feet per minute) you need for your system
Heat Gain (q, in BTU/hr)!
Heat Gain (q, in BTU/hr) = U value x Area, A x (∆T)
Balance Point (Tbalance )
The building balance point temperature is the outdoor air temperature when the heat gains of the building are equal to the heat losses.
Internal heat sources due to electric lighting, mechanical equipment, body heat, and solar radiation may offset the need for additional heating although the outdoor temperature may be below the thermostat set-point temperature. The building balance point temperature is the base temperature necessary to calculate heating degree day to anticipate the annual energy demand to heat a building. The balance point temperature is a consequence of building design and function rather than outdoor weather conditions
• A typical US home uses about ? BTU/year (amount) for space heating
• A typical US home uses about 50 MILLION BTU/year (amount) for space heating
Heating Degree Days (HDD65)
0-1000 no problem, no need to heat
1000 - 3000 good insulation is enough
3000 - 5000 moderate, use 2 systems
5000 - 7000 need serious heating
Over 7000 year long heating (just move)
• Ex: Phoenix HDD65 = 444, Portland = 4693, and Anchorage = 10825
CFM temperature rise method
CFM = BTUHoutput / (1.08 x DeltaT)
