HVAC Flashcards
What are the five parts of the HVAC system?
motors, ducts, fans, controls, and heat exchange units
Purpose of the HVAC system is to add or remove __ and __ to order to maintain the desired environmental conditions
heat and moisture
Summer
Cooling system is used to remove heat from air
this is usually accompanied by removing moisture
sources of cooling: Chiller, packaged unit or heat pump
Winter
heating system is used to add heat to the air
this is usually accompanied by adding moisture
sources of heat; gas fired heater, boiler or heat pump
Most cooling equipment uses a vapor compression cycle and phase changes to transfer heat
chillers, package units, split-units, fridges, freezers, etc.
Vapor Compression Cycle
What are the four main components of the vapor compression cycle?
Compressor, condenser, expansion valve, evaporator
Vapor Compression cycle __ the pressure and the temperature of refrigerant as it goes around the cycle
raises
A ton is a measure of power
One ton of air conditioning = 12,000 Btu/hr
A ton-hour is a measure of energy
One ton-hour of air conditioning = 12,000 Btu
12,000 Btu/hour is the power needed to melt __ __ of ice
one tone
Applies to both heating and cooling
= Energy moved
Energy input
= Power heat flow
Power input
The most important parameter to be measured
As the temperature changes, so does this
Coefficient of Performance (COP) =
Cooling equipment performance = Btu/hr of output / W of electric input = COP * 3.412Btu/ Wh = EER / 3.412 Btu Wh
EER
kWin/ ton = 12/ EER = 3.517/COP
Coefficients for determining equipment performance
predicts chiller efficiency at the Air Conditioning Refrigeration Institute’s (ARI) Standard Rating Point.
(AHU) and (FCU)
Integrated Part Load Value
A typical __ system sends __ water through a tall building to transfer cooling and then the air handler changes water to air to cool each space
chiller
__ __ works to evaporate water into the atmosphere
Each pound of liquid evaporated will send 1,000 Btu of heat to the atmosphere
Cooling Towers
Uses heat instead of electric power to drive a refrigeration cycle
can use waste heat but high maintenance
Absorption Chiller
Oil free variable speed compressor that can be installed as a retrofit to an existing chiller
magnetic bearing centrifugal chillers
Chilled/ hot water is circulated in the coil of the AHU, where a fan blows air to the duct system
allows control of the mix of fresh air and return air
ducted distribution systems
supplies at the same volume and temperature to the entire zone.
single zone systems
can be at different different temperatures OR flow rates
air is mixed and conditions within the machanical room adn then ducted to each zone
not used today
multizone systems
supplies both hot and cold air in separate ducts which are blended at the point of use to provide required conditions
inefficient and expensive
Dual duct systems
air is cooled to the lowest required temperature
temp is controlled by reheat coil
temperature is controlled by controlling the volume of air that is discharged into the space
energy efficient and widely used today
VAV system variable air
small localized air handler unit which serves a single space
a fan in the unit draws room air and blows it across a coil of hot water or chilled water and back into the room
fan coil systems
ventilation requirements reduce carbon dioxide concentration so that it doesn’t exceed ___ ppm
1,000
whenever the outside air is cooler than the cooling set point temperature only distribution energy is required to provide cooling with outside air
return damper is closed 100% so that al return is exhausted and 100% air introduced without cooling
happens in thermally heavy buildings where internal loads are high
economizer cycle
precondition the outdoor make up air with the exhaust air
less inefficient
Energy Recovery ventilators
recover heat from ~200 degree hot refrigerant gas exiting the compressor
domestic cole water (50-60 degrees) can be heated to 140-160 degrees)
Heat available to recover is about 2,500 Btu/h (per ton capacity of an air to air heat pump or cooled air chiller)
HRU - DeSuperheater
Used in CRF and Chilled Beam applications and some VAV types
treats outdoor air to meet ventilation requirements
sized to handle the latent loads
generally include heat recovery wheels with latent capacity
DOAS
simultaneous cooling and heating - basically exchange heat from one room to another.
Multiple units used to expand capacity - condensing units are controlled by at VFD. Evaporators vary on style and locations
VRF
signficantly reduces fan energy and cooling costs. chilled water clws through the coil and air around the coil is cooled and falls into the room
chilled beams
chilled water supply mounted in box. Chilled water supply should be above room dewpoint. Requires a separate air supple that takes care of ventilation and latent requirements
passive chilled beam
compared to traditional VAV, uses reduced amount of pre-treated supply air and then further conditions the air within the occupant space
active chilled beams
the energy required to change the temperature of a substance with no phase change
for air it refers to changing the temperature without changing the moisture content
sensible heat
the energy required to change the phase of a substance
for air, it refers to the energy required from the change of moisture content
latent heat
enthalpy
combination of sensible and latent heat
q = mCpDeltaT
q = CFM * .075lb/ft^3 * 60 min/hour * .24 Btu/lb Degree * Change in temperature
q = CFM * 1.08 * deltaT
Air sensible heat transfer equation
ft^3/minute
CFM
M =
overall mass flow rate of air
Cp
the specific heat (0.24 Btu / lb Degree) for air
q = mCpdelta T
= GPM * 500 * change in temperature
= GPM * 8.34lb/gallon * 60 min/ hour * 1Btu/1lb Degree Fahrenheit
water heat sensible heat transfer equation
q = m*delta h q = CFM * .075lb/ft^3 * 60min/hour * change in enthalpy q = CFM * 4.5 * change in enthalpy
Air sensible and latest heat transfer equation
change temperature without adding moisture (humidity ratio remains constant)
Sensible heating and cooling
increase temperature with addition of moisture to maintain desired relative humidity
heating and humidification
decrease temperature with with reduction of moisture through condensation (humidity ratio decreases)
cooling and dehumidification
Reduce temperature by evaporating into some water, usually effective in hot and dry conditions (humidity ratio increases)
Evaporative Cooling
momentum or resistance to change
A property of the mass of a building which enables it to store heat, providing “intertia” against temperature fluctuations
Thermal Mass
buildings interior conditions respond quickly to weather
vs
interior conditions respond slowly to weather
thermally light buildings vs thermally heavy buildings
infiltration/ventilation
conduction
solar gain
internal loads
Heat Gain
infiltration/ventilation
conduction
heat loss
heat is ___ through the building envelope.
is the inverse of resistance
heat lost/gain through __ depends on: exposed surface area, temperature difference, wall/roof/window construction
imagine standing inside and walking through the walk
q = area* change in temp
sum of R-values of different materials
heat condution
= L/K
or
= L/C
R =
Conductivity “k”
L = thickness in inches
k Units = Btu * in / hr * ft^2 * F
Conductance “C”
C = conductance
on exposed surfaces
we also have to account for the convection that occurs on the material surface
heat convection
U Value for heat transfer
U-Value
U = 1 / Sum of R values
Heat flow equation
q = U * A * Change in Temp
UA is the value of. the product and is often referred to as the Building Load Coefficient
an approximation for geographic weather
Used to predict the amount of heating and cooling needed
average building has desired temp of 70 degrees
5 degrees is supplied by internal heat
**common base for computing degree days is 65 degrees fahrenheit
Degree Days
Q = 24U * A * DD
energy __ loss through the envelope
to get fuel consumption divide by COP or the heating systems efficiency
Seasonal (heat or cool) energy losses through the envelope
\_\_ \_\_ \_\_ Impacts depends on a number of factors: solar radiation intensity time of day orientation availability of shading type of glass
Solar heat gain
the ratio of transmitted solar radiation to incident solar radiation
less is better
solar heat gain coefficient
__ __ needs high solar reflectance and thermal emissivity
the higher the solar reflectance, the __ the surface
Cool roofs
cooler
