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Mechanical Flashcards

1
Q

What is a BTU

A

British Thermal Unit: the amount of heat to raise the temperature of 1 lbm of water by 1deg F.

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2
Q

what is the coefficient of heat transmission?

A

the overall rate of heat flow through any combination of materials, including air.

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3
Q

What is Conductance?

A

the number of BTU’s per hour that pas through 1 SqFt of homogeneous material of a given thickness when the temp differential is 1deg. (for SI units it = rate in watts at which heat passes through 1SqM)

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4
Q

What is Conductivity?

A

the number of BTU’s per hour that pass through 1SqFt of homogeneous material 1 inch thick when the temp differential is 1deg F.

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5
Q

What is the Dew Point?

A

The Temperature at which water vapor is the air becomes saturated & begins to condense to drops of water,

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6
Q

what is the Dry Bulb Temperature?

A

The Temperature of the air-water mixture as measured with a standard dry-bulb thermometer.

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7
Q

What is Isolation (thermal)?

A

the total solar radiation on a horizontal surface.

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8
Q

What is Latent Heat?

A

heat that causes a change of state of a substance, such as the heat required to change water into steam.

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9
Q

What is Resistance (thermal)?

A

The number of hours needed for 1 BTU to pass through 1 SqFt of material or assembly of a given thickness when the temperature differential is 1degF.

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10
Q

What is Sensible Heat?

A

heat that causes a change in Temperature of a substance but not change of state. example: sensible heat req’d to raise the temperature of 1cubic foot of water from 50deg to 100deg is 50 BTUs.

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11
Q

What is Specific Heat?

A

the number of BTU’s (jules) req’d to raise the temperature of a specific material by 1deg F. Specific Heat is a measure of materials capacity to store heat as compared with the storage capacity of water.

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12
Q

what are 4 ways the human body looses heat?

A

CONVECTION: transfer of heat through the movement of fluid, either gas or liquid. eg when the air temp surrounding a person is less than their skin temp
EVAPORATION: occurs when moisture changes to a vapor as a person perspires or breathes.
RADIATION: is the transfer of heat energy through electromagnetic waves from one surface to a cooler surface.
CONDUCTION: is the transfer of heat through direct contact between two objects of different temps.

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13
Q

what is the BTU equivalent of a met?

A

18.4 BTU/ hr

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14
Q

how many Jules are in a BTU?

A

1.055

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15
Q

what direction does heat always flow from?

A

high to low

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16
Q

when surrounding temps are low (vs body temp) how does the body loose heat?

A

Looses temp via convection

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17
Q

when surrounding temps are higher (vs body temp) how does the body loose heat?

A

looses temp via evaporation (sweat)

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18
Q

what is the general (temperature) comfort range for a human?

A

69deg & 80deg

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19
Q

what does the dry ball temp measure?

A

the air temperature

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20
Q

what does the web bulb temp measure?

A

measuers humidity in the air

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21
Q

what is relative humidity?

A

its the ratio of the percentage of moisture in the air to the maximum amount air can hold at a given temp. without condensing.

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22
Q

what does air movement contribute to?

A

Air movement tends to increase evaporation and heat loss through convection.

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23
Q

what is the general tolerability limits for humidity?

A

20% to 75%

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24
Q

what is enthalpy?

A

the total amount of both sensible and latent heat in the air-moisture mixture. It tells us the total amount of heat that must be removed (cooling) or added (in heating) from conditioned air. Its graphed on the pschometric chart

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25
Q

what is convection?

A

transfer of heat through movement of air (fluid). ie Hot air rising and cool air falling.

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26
Q

what is conduction?

A

transfer of heat via direct contact between molecules.

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27
Q

what is radiation?

A

transfer of heat energy through electromagnetic waves. Think a fire place.

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28
Q

what does ‘R’ measure?

A

R= Resistance, its the number of hours needed for 1 btu to pass through a material of a given thickness when the temp differential is 1 deg F.

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29
Q

what does ASHRAE stand for?

A

American Society of Heating, Refrigeration and Air Conditioning Engineers.

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30
Q

What dies ‘U’ measure?

A

the measurement that calculates the coefficient of heat transmission,

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31
Q

How do you calculate the conversion of ‘R’ to ‘U’?

A

U= 1/R

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32
Q

How do you calculate the conversion of Conductance (‘C’) into ‘R’ (resistance)

A

R= 1/C

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33
Q

what is the average heat gain of an occupant (office) at rest?

A

400 Btu/Hr

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34
Q

what is sensible heat gain?

A

Sensible heat is heat exchanged by a body or thermodynamic system that changes the temperature. Its assumed to be 225 Btu/Hr per occupant. Its basically the heat gained from occupants of a building.

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35
Q

what is latent heat gain?

A

Latent heat is the energy released or absorbed by a body or a thermodynamic system during a constant-temperature process.

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36
Q

what is the heating value of natural gas & propane?

A

Gas: 1050 Btu/ft3
Propane: 2500 Btu/ft3

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37
Q

heating oil is rated how?

A

grades: 1 threw 6, lower the number the more refined.

38
Q

what is the general heating value of heating oil?

A

~137,000 Btu/ Gal

39
Q

What is the heating equivalent for electricity?

A

3413 Btu/kW

40
Q

what is a degree day?

A

its a measure of the approximate average yearly temperature difference between the outside & inside in a particular location.

41
Q

what is compressive refrigeration?

A

its based upon the transfer of heat during the liquefaction and evaporation of a refrigerant. As a refrigerant in a gas form is compressed, it liquefies and releases latent heat as it changes state. As the liquid expands and vaporizes back to a gas, it absorbs latent heat from the surroundings into the gas.

42
Q

what are the 3 major components of a compression refrigeration cycle?

A

COMPRESSOR: takes the refrigerant in a gas form and compresses it to a liquid
CONDENSER: The coil where the latent heat is released.
EVAPORATOR: Where the liquid is expanded (back to a gas) and the latent heat is absorbed.

43
Q

What is refrigeration by absorption?

A

produces chilled water & is accomplished by the loss of heat when water evaporates. This evaporation is produced in a closed system by a weak salt solution that draws water vapor from the evaporator.

44
Q

what is 1 ton of refrigeration mean?

A

its the cooling effect obtained when 1 ton of 32deg ice melts to water at 32deg F in 24hr. Its roughly equivalent to 12,000 Btu/hr. In general the required capacity of a refrigeration system can be determined by dividing the total heat gain by 12k.

45
Q

what is a Direct Expansion (DX) system?

A

passes non ducted air to be cooled over the evaporator and back into the room (think residential units).

46
Q

what is an All-Air System?

A

it heats or cools spaces by conditioned ir alone. Heat is transported to the space with the supply & return air ducts. The most basic type of this system is the constant
volume single duct.

47
Q

what is a dual duct (high velocity) system?

A

Basically two ducts go to each room/ zone (one cool air, & one hot air) and there is a mixing box that attaches to a thermostat that merges the two types of air to get precise temp. This system can respond well to dramatic changes in temps throughout the building.

48
Q

what is a reheat (constant volume) system?

A

the system takes return air & fresh outdoor air & cools & dehumidifies the mix which is then distributed in a constant volume. The re-heat portion takes place at the space.

49
Q

what is the suggested area size that should be set aside for mechanical rooms?

A

3% to 9% of the gross building area.

50
Q

what is a displacement ventilation system?

A

supply air is fed from the floor below (raised access floor) and the return is in the ceiling.

51
Q

what is heat flow?

A

heat the tends to flow from a hot area to a cool area.

52
Q

what is evaporation?

A

as a liquid vaporizes it takes heat from the surface along with it

53
Q

what is absolute humidity?

A

The amount of water vapor present in a unit volume of air, usually expressed in grams per cubic meter

54
Q

what is mean radiant temp?

A

add up the temp of the 4 walls in the room an divide by 4 for the average.

55
Q

how do you measure the heat that a light fixture puts off into a space?

A

measured in BTU/hr = 3.41 x Watts.

eg a 100 watt light bulb puts off 341 BTU/hr.

56
Q

how do you measure the heat that a motor puts off into a space?

A

2145 x horsepower of the equipment

57
Q

what is the shading coefficient of glass mean?

A

s a value that determines one type of thermal performance of a glass unit (panel or window) in a building. It is basically the ratio of solar gain (due to direct sunlight) passing through a glass unit to the solar energy which passes through 1/8” Clear Float Glass

58
Q

what does the expansion valve do in a DX system.

A

is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator.

59
Q

what are the 3 things needed to determine the hourly flow though a building’s envelope?

A
  • The rate @ which heat flows threw various assemblies (Resistance)
  • the are of those assemblies
  • the temp difference between the inside & outside.
60
Q

what is a ‘U’ factor?

A

measures thermal transmittance.

61
Q

what is the SHGC (Solar Heat Gain Coefficient)?

A

) is used in the United States and most commonly refers to the solar energy transmittance of a window or door as a whole, factoring in the glass, frame material (wood, aluminum, etc.), sash (if present), divided lite bars (if present) and screens (if present). SHGC may also refer to the solar energy transmittance of the glass alone (sometimes more specifically termed center-of-glass SHGC), in which case it is analogous to g-value.

62
Q

what is stack effect?

A

Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences.

63
Q

what does a make-up air unit do?

A

these prevent a negative indoor air pressures by forcing air for the air thats exhausted from the process.

64
Q

what types of environments are air to air heat exchangers useful in?

A

cold weather.

65
Q

What is Dessicant Cooling?

A

They lower humidity w/ out having to resort to overcooling air. A sillica is heated to drive out moisture.

66
Q

what on average is the air exchange rate

A

6 to 10 times per hour

67
Q

what is a degree day?

A

Heating Degree Days (HDD) for a particular climate is obtained by subtracting each day’s mean outdoor dry bulb temperature from the balance point temperature; this result is the number of HDDs for that day. For example, if the maximum and minimum outdoor dry bulb temperatures of a place were 80°F and 20°F respectively, and the balance point temperature were 65°F, then HDD of the place for that particular day would have been 65-[(80+20)/2] = 15. If the mean outdoor dry bulb temperature is equal to or higher than the balance point temperature, then the HDD would be equal
to 0.

68
Q

what is the emissivity of an object?

A

it measures its ability to absorb & then radiate heat.

69
Q

what are degree days?

A

Degree Days are used to determine the heating requirements of a building. It’s the
amount by which the average outdoor temp is below 65ºF for one day. The more
days it’s below that, the larger number of degree days it has

70
Q

what is low-e glazing?

A

double glazing with a thin film in the glazing cavity that allows
visible and near infrared to be transmitted
• In cold climates: apply to the inside pane of glass
• In warm climates: apply to the outside pane of glass
• As objects in the room are heated and emit long wave radiation, the film
prevents the loss of this heat by reflecting it back into the room
• When used with argon gas the system is very efficient

71
Q

what is thermal bridging?

A

• A fundamental of heat transfer where a penetration of the insulation layer by a
highly conductive or non-insulting material takes place in the Spartan between the
interior and eater environments of the building enclosure

72
Q

what direction does heat always flow?

A

heat always flows from hot to cold. A heated
gas, such as air, flows upward by convection; however heat will conduct and radiate in
any direction, but always from hot to cold.

73
Q

what are factors involved in sensible heat load?

A

air via ventilation, infiltration, Solar heat gain via glass, lights & appliances, Body heat

74
Q

What approach should be used to most effectively limit low-frequency sounds in an HVAC system?

A

Duct Linings

75
Q

what is an air cushion or expansion tank?

A

its a closed tank containing air , usually located above the boiler. When water in the system is heated, it expands, compressing the air in this tank. This tank facilitates not needed to constantly open & close a relief valve.

76
Q

what is the Energy Efficiency Ratio (EER) ?

A

the ratio of the net equipment cooling capacity in BTU/hr to the total rate of electricity input in watts

77
Q

what is the Coefficient of Performance? (COP)

A

its defined for cooling as the ratio of the rate of heat removal to the rate of energy input in constant units

78
Q

what is SEER?

A

Seasonal Energy Efficiency Ratio: it measures the number of BTU/hr removed for each watt of energy input, averaged over the cooling season.

79
Q

what is a direct refrigerant system?

A

not ducting as the refrigeration cycle is contained within the space it serves. small systems..think hotel rooms

80
Q

curved baffles in ducts are called what?

A

turning vanes

81
Q

what is the comfort zone for humans?

A

65d to 78d 25% to 75%

82
Q

what is the max pitch for an escalator?

A

30deg

83
Q

what is the max pitch for an escalator?

A

30deg

84
Q

what is adiabatic wet-bulb temperature?

A

(ENTHALPY STAYS THE SAME) is the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic

85
Q

how do you calculate degree days (DD) for a specific day?

A

its the difference between an indoor temp of 65deg and the average outside temp for a 24 hr period. eg:
65-(24hr average in deg) = x

86
Q

what does refrigeration by absorption produce?

A

chilled water

87
Q

for larger building what are the 4 types of all-air systems?

A

VAV: one duct and dampers in each zone/ room
DUAL DUCT (high velocity): two parallel ducts w/ mixing box
REHEAT (Constant Volume): one (cool) duct per zone and a re-heat box at the zone ready for when demanded. more efficient than a Dual Duct. All air in a building in a constant velocity and temp, the heat is added as needed.
MULTIZONE: supplies air to a central mixing unit where separate heating and cooling coils produce hot & cold air streams.

88
Q

what is a displacement ventilation system?

A

supply air originates at the floor level & rises to return at the ceiling.

89
Q

what is an Energy Recovery Ventilator Or a heat exchanger?

A

reclaims waste energy from the exhaust air stream & uses it to condition incoming fresh air.

90
Q

why do we insulate air ducts?

A

Reduces heat loss/heat gain of air during delivery
• Isolates air nosies in duct from spaces
• Prevents condensation on outside for cool-air ducts

91
Q

What happens to latent heat when you change a liquid to a gas?

A

its absorbed

92
Q

What happens to latent heat when you change a gas to a liquid?

A

its released

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