objectives learn basics about ahus review thermodynamics - solve thermodynamic problems and use...

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

Cooling coil•Heat transfer from air to refrigerant•Extended surface coil

Drain Pain•Removes moisture condensed from air stream

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

• 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 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 heatΔt = 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 + Pvh = enthalpy (J/kg, Btu/lbm)P = Pressure (Pa, psi)v = specific volume (m3/kg, ft3/lbm)

Second lawIn 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

T

QdS

S = entropy (J/K, BTU/°R)Q = heat (J, BTU)T = absolute temperature (K, °R)

Thermodynamic IdentityUse total differential to H = U + PVdH=dU+PdV+VdP , using dH=TdS +VdP →→ TdS=dU+PdVOr: 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

Kkg

kJ314.8

R

lbf

lbm

ft1545

MMR

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

• 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)

Px V = mx Rx∙TPy V = 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 constantT = 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

• 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

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)