Objectives

• Define basic HVAC and AHU components

• Learn about Psychrometrics

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

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)

Properties of water

• Water, water vapor (steam), ice

• Properties of water and steam (pg 675 – 685)• Alternative - ASHRAE Fundamentals ch. 6

Humidity Ratio, W

• W = mw/ma

• Degree of saturation, µ = W/Ws

• Humidity ratio is hard to measure, but very useful in calculations

• What are units?

• Is W a function of temperature? What about Ws?

Ws = humidity ratio at saturationma = mass of dry airmw = mass of water vapor

Relative Humidity

• Φ = xw/xw,s = Pw/Pws

• Function of T

Easy to measure and useful in some contexts, but often need to know temperature as well

W

Ws

622.0

622.0

x = mole fractionP = pressureμ = degree of saturationW = humidity ratio

Dew-point temperature, td

• Temperature at which condensation will form• Under appropriate surface conditions

• Vapor is saturated

• Φ = ?

• Ws(P, td) = W

Wet-bulb temperature, VBT (t*)

• Temperature of wet surface or

• Temperature at which water, by evaporating into the air, will bring air to saturation adiabatically

• * superscript is designation that variable is evaluated at the wet-bulb temperature

• Note, distinct from that measured by a sling psychrometer• Section 9.5

Tables for Moist Air (P = 1 atm)

• Tables A.4 in your text

• Ability to get Ws for calculations

• Subscripts:• a = dry air, s = saturated air

v = va+µvas

h = ha+µhas

s = sa+µsas

Psychrometric Chart

• Need two quantities for a state point• Can get all other quantities from a state point

• Can do all calculations without a chart• Often require iteration• Many “digital” psychrometric charts available

• Can make your own

• Best source is ASHRAE fundamentals (Chapter 6)• Also in your text (back cover fold-out)

Alternate calculation for W

• PV = mRT (IGL)

• What do we know about R ratio?

• P = Pw + Pa

w

a

a

w

TRVP

TRVP

a

w

R

R

P

P

m

mW

a

a

w

w

w

w

PP

PW

622.0

R = gas constantP = pressureV = volumeT = absolute temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air

Calculation of psychometric quantities

• For an ideal gas,• hda = ∫cpadT, hw = ∫cpwdT

• So, hda = cp,dat which assumes a reference state of 0 °F or 0 °C – Tables A4• Note different reference

• hw = cpwt + hg0

• h = cp,dat + W(cpwt + hg0)Or you can use:• h = cpt + W∙hg0, cp = cp,da + Wcpw

cp = specific heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio

Subscripts: w is water vapor, a is dry air, g is saturated water vapor

Adiabatic mixing

• Governing equation hmQhmoutin

External heat

Sensible heating

tcmQ p

Dehumidification by Cooling

Real Dehumidification Process