Transcript 389H_NO_02_review_I

Objectives
• Learn basics about AHUs
• Review thermodynamics
- Solve thermodynamic problems and use properties
in equations, tables and diagrams
Systems: Heating
• Make heat (furnace, boiler, solar, etc.)
• Distribute heat within building (pipes, ducts,
fans, pumps)
• Exchange heat with air (coils, strip heat,
radiators, convectors, diffusers)
• Controls (thermostat, valves, dampers)
Systems: Cooling
• Absorb heat from building (evaporator or chilled
water coil)
• Reject heat to outside (condenser)
• Refrigeration cycle components (expansion valve,
compressor, concentrator, absorber, refrigerant)
• Distribute cooling within building (pipes, ducts, fans,
pumps)
• Exchange cooling with air (coils, radiant panels,
convectors, diffusers)
• Controls (thermostat, valves, dampers, reheat)
Systems: Ventilation
• Fresh air intake (dampers, economizer, heat
exchangers, primary treatment)
• Air exhaust (dampers, heat exchangers)
• Distribute fresh air within building (ducts,
fans)
• Air treatment (filters, etc.)
• Controls (thermostat, CO2 and other
occupancy sensors, humidistats, valves,
dampers)
Systems: Other
• Auxiliary systems (i.e. venting of combustion
gasses)
• Condensate drainage/return
• Dehumidification (desiccant, cooling coil)
• Humidification (steam, ultrasonic humidifier)
• Energy management systems
Drain Pain
•Removes
moisture
condensed
from air
stream
Cooling coil
•Heat transfer
from air to
refrigerant
•Extended
surface coil
Condenser
Expansion valve
Controls
Compressor
Heating coil
•Heat transfer
from fluid to
air
Heat pump
Furnace
Boiler
Electric resistance
Controls
Blower
•Overcome
pressure drop
of system
Adds heat to air
stream
Makes noise
Potential hazard
Performs
differently at
different
conditions (air
flow and
pressure drop)
Duct system
(piping for
hydronic
systems)
•Distribute
conditioned
air
•Remove air
from space
Provides
ventilation
Makes noise
Affects comfort
Affects indoor air
quality
Diffusers
•Distribute
conditioned
air within
room
Provides
ventilation
Makes noise
Affects comfort
Affects indoor air
quality
Dampers
•Change
airflow
amounts
Controls outside
air fraction
Affects building
security
Filter
•Removes
pollutants
•Protects
equipment
Imposes
substantial
pressure drop
Requires
Maintenance
Controls
•Makes
everything
work
Temperature
Pressure (drop)
Air velocity
Volumetric flow
Relative humidity
Enthalpy
Electrical Current
Electrical cost
Fault detection
Review
• Basic units
• Thermodynamics processes in HVAC systems
Units
• Pound mass and pound force
• lbm = lbf (on Earth, for all practical purposes)
• Acceleration due to gravity
• g = 9.807 m/s2 = 32.17 ft/s2
• Pressure (section 2.5 for unit conversions)
• Temperature (section 2.6 for unit conversions)
Thermodynamic Properties
• ρ = density = mass / volume
Both functions of t, P
• v = specific volume = 1 / ρ
• specific weight = weight per unit volume
(refers to force, not to mass)
• specific gravity = ratio of weight of volume of
liquid to same volume of water at std.
conditions (usually 60 °F or 20 °C and 1 atm)
Heat Units
• Heat = energy transferred because of a
temperature difference
• Btu = energy required to raise 1 lbm of water 1 °F
• kJ
• Specific heat (heat per unit mass)
• Btu/(lbm∙°F), kJ/(kg∙°C)
• For gasses, two relevant quantities cv and cp
• Basic equation (2.10) Q = mcΔt
Q = heat transfer (Btu, kJ)
m = mass (kg, lbm)
c = specific heat
Δt = temperature difference
Sensible vs. latent heat
• Sensible heat Q = mcΔt
• Latent heat is associate with change of phase
at constant temperature
•
•
•
•
Latent heat of vaporization, hfg
Latent heat of fusion, hfi
hfg for water (100 °C, 1 atm) = 1220 Btu/lbm
hfi for ice (0 °C, 1 atm) = 144 Btu/lbm
Work, Energy, and Power
• Work is energy transferred from system to
surroundings when a force acts through a
distance
• ft∙lbf or N∙m (note units of energy)
• Power is the time rate of work performance
• Btu/hr or W
• Unit conversions in Section 2.7
• 1 ton = 12,000 Btu/hr (HVAC specific)
Where does 1 ton come from?
•
•
•
•
•
•
1 ton = 2000 lbm
Energy released when 2000 lbm of ice melts
= 2000 lbm × 144 BTU/lbm = 288 kBTU
Process is assumed to take 1 day (24 hours)
1 ton of air conditioning = 12 kBTU/hr
Note that it is a unit of power (energy/time)
Thermodynamic Laws
• First law?
• Second law?
• Implications for HVAC
• Need a refrigeration machine (and external energy)
to make energy flow from cold to hot
Internal Energy and Enthalpy
• 1st law says energy is neither created or destroyed
• So, we must be able to store energy
• Internal energy (u) is all energy stored
• Molecular vibration, rotation, etc.
• Formal definition in statistical thermodynamics
• Enthalpy
• Total energy
• We track this term in HVAC analysis
• h = u + Pv
h = enthalpy (J/kg, Btu/lbm)
P = Pressure (Pa, psi)
v = specific volume (m3/kg, ft3/lbm)
Second law
In any cyclic process the entropy will either increase or
remain the same.
Entropy
• Not directly measurable
• Mathematical construct
Q
dS 
T
S = entropy (J/K, BTU/°R)
Q = heat (J, BTU)
T = absolute temperature (K, °R)
• Note difference between s and S
• Entropy can be used as a condition for equilibrium
Thermodynamic Identity
Use total differential to H = U + PV
dH=dU+PdV+VdP , using dH=TdS +VdP →
→ TdS=dU+PdV
Or: dU = TdS - PdV
T-s diagrams
• dH = TdS + VdP (general property equation)
• Area under T-s curve is change in specific energy
– under what condition?
T-s diagram
h-s diagram
p-h diagram
Ideal gas law
• Pv = RT or PV = nRT
• R is a constant for a given fluid
• For perfect gasses
• Δu = cvΔt
• Δh = cpΔt
• cp - cv= R
1545 ft lbf 8.314 kJ
R

M lbm R
M kg  K
M = molecular weight (g/mol, lbm/mol)
P = pressure (Pa, psi)
V = volume (m3, ft3)
v = specific volume (m3/kg, ft3/lbm)
T = absolute temperature (K, °R)
t = temperature (C, °F)
u = internal energy (J/kg, Btu, lbm)
h = enthalpy (J/kg, Btu/lbm)
n = number of moles (mol)
Mixtures of Perfect Gasses
•
•
•
•
•
m = mx my
Px V = mx R ∙T
V = Vx Vy
Py V = my R ∙T
T = Tx Ty
What is ideal gas law for mixture?
P = Px Py
Assume air is an ideal gas
x
y
• -70 °C to 80 °C (-100 °F to 180 °F)
m = mass (g, lbm)
P = pressure (Pa, psi)
V = volume (m3, ft3)
R = material specific gas constant
T = absolute temperature (K, °R)
Enthalpy of perfect gas mixture
• Assume adiabatic mixing and no work done
• What is mixture enthalpy?
• What is mixture specific heat (cp)?
Mass-Weighted Averages
• Quality, x, is mg/(mf + mg)
• Vapor mass fraction
• φ= v or h or s in expressions below
• φ = φf + x φfg
s = entropy (J/K/kg, BTU/°R/lbm)
m = mass (g, lbm)
• φ = (1- x) φf + x φg
h = enthalpy (J/kg, Btu/lbm)
v = specific volume (m3/kg)
Subscripts f and g refer to saturated
liquid and vapor states and fg is the
difference between the two
Properties of water
• Water, water vapor (steam), ice
• Properties of water and steam (pg 675 – 685)
• Alternative - ASHRAE Fundamentals ch. 6
Psychrometrics
•
•
•
•
What is relative humidity (RH)?
What is humidity ratio (w)?
What is dewpoint temperature (td)?
What is the wet bulb temperature (t*)?
•
•
•
•
How do you use a psychrometric chart?
How do you calculate RH?
Why is w used in calculations?
How do you calculate the mixed conditions for two
volumes or streams of air?
Thermodynamic Properties of
Refrigerants
• What is a refrigerant?
• Usually interested in phase change
• What is a definition of saturation?
• ASHRAE Fundamentals ch. 20 has additional
refrigerants
Homework Assignment 1
• Review material from chapter 2
• Mostly thermodynamics and heat transfer
• Depends on your memory of thermodynamics and
heat transfer
• You should be able to do any of problems in
Chapter 2
• Problems 2.3, 2.6, 2.10, 2.12, 2.14, 2.20, 2.22
• Due on Thursday 2/3 (~2 weeks)