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# Objectives PowerPoint PPT Presentation

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)

Objectives

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### Objectives

• 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

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 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

• 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

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?

### 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

• 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

• 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)