SYSTEMS - MECHANICAL PDD Flashcards
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
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
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.
• Poor Indoor Air Quality gets upgraded to Sick Building Syndrome when:
10% of occupants are sick
20% of occupants complain
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
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)
