Objectives
Download
1 / 36

Objectives - PowerPoint PPT Presentation


  • 70 Views
  • Uploaded on

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)

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Objectives' - harlan-paul


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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


  • Drain Pain

  • Removes moisture condensed from air stream

  • Cooling coil

  • Heat transfer from air to refrigerant

  • Extended surface coil

Condenser

Expansion valve

Controls

Compressor


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)


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


Controls outside air fraction

Affects building security


  • Filter

  • Removes pollutants

  • Protects equipment

Imposes substantial pressure drop

Requires Maintenance


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?





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)


ad