THERMAL PROCESSES Flashcards
In a thermal process the emissivity of a surface is a property of the material and usually the same as the _____ any given wavelength?
Absorptivity
What is an HTM?
A heat transfer multiplier
In terms of emissivity and absorptivity, what is the difference between black surfaces and shiny surfaces?
Black surfaces heat up quickly and cool off quickly. Shiny surfaces heat up more slowly but stay hot longer.
What is a selective surface?
A material with a high absorptivity in one wavelength (usually solar) and low emissivity in another (usually infrared). This means the material stores incoming solar radiation without releasing it at infrared i.e. a good solar collector panel.
What is transmissivity?
The measure of how easily a material allows radiant energy to pass through it.
What is the difference between the transmissivity of glass in the visible spectrum and the transmissivity of glass in the infrared region?
Glass has a high transmissivity in the visible spectrum, but a low transmissivity in the infrared region and this causes the greenhouse effect
Infrared wavelengths are _____ while visible spectrum wavelengths are _____.
LONG infrared and SHORT visible
What is the greenhouse effect?
Mostly visible spectrum, short wavelength, energy comes through glass. It then warms objects in the room, and is reradiated as infrared heat. But it that point it can’t escape back out to the class.
The viewed angle of a surface depends on what two things?
The size of the surface and your distance from it… Think about standing next to a freezer and then think about standing very far away from it… Different implications in heat loss
What is MRT?
Mean radiant temperature is the average radiant temperature of your surroundings.
What’s the only thing that can measure MRT?
The globe thermometer
In convection, what is stagnation? How is that different from the stack effect?
When the top of the space is warmer than the bottom, and the hot air rises and stays there.
The stack effect is more specifically about the difference in pressure in a vertical space (positive or outward) at the top and negative (or inward) at the bottom.
What is the film coefficient?
That relates to the thin film of their which occurs next to a wall, which provides a resistance.
What is the inverse of conductivity?
Resistivity
What is the formula for calculating resistance?
x/k = R
Where:
R is resistance
x is the thickness
k is conductivity
What does it mean when the resistance of a material is designated R-19?
This simply means that the material has a resistance of 19 (ft.²°F hr/BTU)
If resistance is noted with an R, what is the letter symbolizing conductance? And what is the formula for it?
U = 1/(R_1 + R_2 + R_3 + … + R_n)
Because the U value is the reciprocal of the sum of all of the resistances in a wall assembly
What is a eutectic salt for?
It’s a phase change material
How many BTUs does it take to change 1 pound of water 1°F?
One BTU
How many BTUs does it take to get 1 pound of water from freezing to boiling?
212°F -32°F equals 180°F… Therefore that’s 180 BTU
What is the latent heat of Apparation and what does this mean?
1000 BTUs, and it means how much energy it takes to boil a liquid or whatever
What is a degree day?
A measure of how cold it has been at a given place over a given period of time
What is the formula that describes conductance there were surface area at a given point in time? Rework this formula for a flow rate over a long period of time.
Conductance q_c = U * A * deltaT
U is U factor
A is area of surface
DeltaT is change in temp
Conductance over period of time:
q_c = U * A * 24(DD)
Where DD is degree days
What is the design day?
It’s a day that represents the most extreme to design a system for. It represents a day colder than 98% of any other day
What is the payback period?
Is the number of years it would take to recoup the investment, for instance on increased insulation, in terms of energy costs.
What are the two things you must do in order to calculate infiltration load or q_i?
Determine the amount of air infiltration.
Determine the amount of heating or cooling required to bring the air to the proper temperature.
What are the two methods of determining the amount of air infiltration?
The air change method. You have to know the air changes per hour. Expressed in cubic ft per hr;
Q_cfh = N x V
Where:
N is the # of air changes
V is the building volume
The crack method. Based on the number of linear feet of crack or joint in all of the windows in the space under consideration.
How many BTUs per hour does the average person at rest generate?
450 BTUs per hour
What is the CLTD?
Ruling the temperature differential. It has to do with the fact that peak games usually current sunny weather when there’s a large date tonight temperature swing.
What is insolation?
Radiant gain through glass
What is enthalpy?
Th combined energy stored in the air including sensible and latent heat
What do the curved lines represent in a psychrometric chart?
Relative Humidity
What is the definition of relative humidity?
The percentage of water in the air at a given temperature compared to how much water the air could hold at that temperature…
Ratio of humidity to complete saturation at a given temperature
Air can hold more water when it is _____
Warm
Always put the vapor barrier on which side of the insulation?
Warm side
What is the comfort range your comfort zone, in terms of temperature and relative humidity?
65° to 78°… 25 RH to 75 RH
What factor does the psychrometric chart not include, which affects effective temperature?
MRT… If the MRT is high you need a lower ambient temp for comfort and vice versa
It is the declination angle on the winter solstice versus the summer solstice?
On June 21st it is +23.5°. December 21 it is -23.5°.
What is the difference between legislated time and sidereal time?
Sidereal time (real or solar time) only exists at the center of the time zone. Real or solar time is in actuality either earlier or later than the legislative time.
The angle of the shadow line is called the ____ ____
Profile angle
What is the solar plot? And what is the shadow mask?
The path of the sun plotted onto a grid. Shadow mask is the representation of shading devices plotted on to that same grid.
What is direct normal intensity?
The highest intensity when a hypothetical receiving surface is exactly perpendicular to the solar vector … From an angle there should be a larger elliptical area of light at a lower intensity
What is dew point?
When the temperature lowered below the temperature at which saturation occurs for the humidity in the air …
Which climate had the greatest daily temperature variations? Why?
Hot arid… Usually there is a clear sky and at night there are high radiation losses to the sky … And evaporation happens easily
What is the strict definition of a passive system?
When in which the collector device in the storage device are one in the same, i.e. the structure itself
How many times more thermal mass is required in an indirect gain space compared to a direct gain space to have the same effect in heat gain?
About 4 more times
What are typical R values on walls and roof for a super insulated building?
R40 in the walls, R60 on the roof
How low do you have to go underground to find a spot where the temperature doesn’t change at all
About 20 feet
What’s different about a Fresnel lens?
Basically serrated, it uses less material than a normal length, sometimes a bent Fresnel lens is part of a focusing collector
What’s the difference between an open loop and a closed loop system?
In an open loop system the food going to the Loop is the fluid that will be consumed. But a closed loop system uses one medium to transfer heat through a coil to another.
What does a closed loop system solve?
It replaces water, which might freeze and then expand and burst pipes
A breadbox system is the name for a ____
Batch system – very simple storage tank filled with water in the sun
Describe an air and rock bed storage system.
Warm air from a solar collector, a greenhouse base, etc., is passed through ducks to a large bin full of rocks usually underneath the house. This warm air deposits heat in those rocks and later you can blow in reverse and get heat in the house
Describe desiccant cooling.
The sun bakes all the moisture out of the desiccant. Then air is passed over it and it collects the moisture out of that air, cooling it.
What are typical efficiencies of photovoltaic cells?
10% to 13%
What are the two basic types of Wind turbine?
Vertical axis Wind turbine and horizontal axis wind turbine
What are the two types of VAWT?
The darrieus which looks like an eggbeater stuck in the ground. More efficient.
The savonius which looks like two half cylinders. Self-starting.
