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Objectives

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- Learn basics about AHUs
- Review thermodynamics
- Solve thermodynamic problems and use properties in equations, tables and diagrams

- 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)

- 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)

- 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)

- 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

- Basic units
- Thermodynamics processes in HVAC systems

- 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)

- ρ = density = mass / volume
- 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)

Both functions of t, P

- 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 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 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)

- 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)

- First law?
- Second law?
- Implications for HVAC
- Need a refrigeration machine (and external energy) to make energy flow from cold to hot

- 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)

In any cyclic process the entropy will either increase or remain the same.

Entropy

- Not directly measurable
- Mathematical construct
- Note difference between s and S

- Entropy can be used as a condition for equilibrium

S = entropy (J/K, BTU/°R)

Q = heat (J, BTU)

T = absolute temperature (K, °R)

Use total differential to H = U + PV

dH=dU+PdV+VdP , using dH=TdS +VdP →

→ TdS=dU+PdV

Or: dU = TdS - PdV

- dH = TdS + VdP (general property equation)
- Area under T-s curve is change in specific energy – under what condition?

- Pv = RT or PV = nRT
- R is a constant for a given fluid
- For perfect gasses
- Δu = cvΔt
- Δh = cpΔt
- cp - cv= R

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)

- m = mx my
- V = Vx Vy
- T = Tx Ty
- P = Px Py
- Assume air is an ideal gas
- -70 °C to 80 °C (-100 °F to 180 °F)

PxV = mx Rx∙T

PyV = my Ry∙T

What is ideal gas law for mixture?

m = mass (g, lbm)

P = pressure (Pa, psi)

V = volume (m3, ft3)

R = material specific gas constant

T = absolute temperature (K, °R)

- Assume adiabatic mixing and no work done
- What is mixture enthalpy?
- What is mixture specific heat (cp)?

- Quality, x, is mg/(mf + mg)
- Vapor mass fraction

- φ= v or h or s in expressions below
- φ = φf + x φfg
- φ = (1- x) φf + x φg

s = entropy (J/K/kg, BTU/°R/lbm)

m = mass (g, lbm)

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

- Water, water vapor (steam), ice
- Properties of water and steam (pg 675 – 685)
- Alternative - ASHRAE Fundamentals ch. 6

- 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?

- What is a refrigerant?
- Usually interested in phase change

- What is a definition of saturation?
- ASHRAE Fundamentals ch. 20 has additional refrigerants

- 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)