Objectives
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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)

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Objectives

Objectives

  • Learn basics about AHUs

  • Review thermodynamics

    - Solve thermodynamic problems and use properties in equations, tables and diagrams


Systems heating

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

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

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

Systems: Other

  • Auxiliary systems (i.e. venting of combustion gasses)

  • Condensate drainage/return

  • Dehumidification (desiccant, cooling coil)

  • Humidification (steam, ultrasonic humidifier)

  • Energy management systems


Objectives

  • Drain Pain

  • Removes moisture condensed from air stream

  • Cooling coil

  • Heat transfer from air to refrigerant

  • Extended surface coil

Condenser

Expansion valve

Controls

Compressor


Objectives

  • Heating coil

  • Heat transfer from fluid to air

Heat pump

Furnace

Boiler

Electric resistance

Controls


Objectives

  • 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)


Objectives

  • Duct system (piping for hydronic systems)

  • Distribute conditioned air

  • Remove air from space

Provides ventilation

Makes noise

Affects comfort

Affects indoor air quality


Objectives

  • Diffusers

  • Distribute conditioned air within room

Provides ventilation

Makes noise

Affects comfort

Affects indoor air quality


Objectives

  • Dampers

  • Change airflow amounts

Controls outside air fraction

Affects building security


Objectives

  • Filter

  • Removes pollutants

  • Protects equipment

Imposes substantial pressure drop

Requires Maintenance


Objectives

  • Controls

  • Makes everything work

Temperature

Pressure (drop)

Air velocity

Volumetric flow

Relative humidity

Enthalpy

Electrical Current

Electrical cost

Fault detection


Review

Review

  • Basic units

  • Thermodynamics processes in HVAC systems


Units

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

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 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 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, 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

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

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

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

Second law

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)


Thermodynamic identity

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

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

T-s diagram


H s diagram

h-s diagram


P h diagram

p-h diagram


Ideal gas law

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

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

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)

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)


Enthalpy of perfect gas mixture

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

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

Properties of water

  • Water, water vapor (steam), ice

  • Properties of water and steam (pg 675 – 685)

    • Alternative - ASHRAE Fundamentals ch. 6


Psychrometrics

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

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

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)


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