HVAC Flashcards
Heat Transfer - Uniform Material
Q = (k A DT ) / t
Heat Transfer - Pipe
Q = (k 2pi L DT) / ln (r2 / r1)
Heat Transfer Coefficient
U = (U1 A1 + U2 A2) / A U = 1 / R = k / t R = (1 / h) + R1 + R2 R1 = L / k
Heat Transfer - k
k - conductivity, Btu in/ hr ft2 F
Molar Mass Air & Water
Mair = 29 g/mol Mw = 18 g/mol
Specific Heat Air & Water
Cpair = .24 BTU/lb F Cpw = 1 BTU/lb F
Density Air & Water
Pair = .075 lb/FT3 Pw = 62.4 lb/FT3
Heat Transfer - Water
Q (BTU/h) = 500 Q DT
Q = m Cp DT
Heat Transfer - Sensible
Q (BTU/h) = 1.1 Q DT
Heat Transfer - Latent
Q (BTU/h) = 4,840 Q DW
Heat Transfer - Total
Q (BTU/h) = 4.5 Q DH
Mixing Airstreams
h3 = h1 Q1 + h2 Q2 / (Q1 + Q2) w3 = w1 Q1 + w2 Q2 / (Q1 + Q2) T3 = T1 Q1 + T2 Q2 / (Q1 + Q2) h3 = h1 RH1 + h2 RH2
What causes condensation?
Surface T is below DP T
Low P refrigerants _____ at _____ T
boil low
High P refrigerants _____ at _____ T
condense high
Evaporation
Low P (constant), High T (constant - all heat changes phase), Gas
Compressor
High P (inc), High T, Gas (constant entropy)
Condenser
High P, Low T, Liquid
Expansion Valve
Low P, Low T, Gas/Liquid
BHP vs HP
HP = BHP / Eff(m)
Enthalpy of mixtures
h(fg) = h(f) - h(g) h(mix) = h(f) + X h(fg)
Isentropic
constant entropy (s)
Adiabatic
no heat transfer (q=0)
q (heat output for motor)
q (btu/h) = HP * 2545 * Eff(m)
Cooling Tower - Range
DT across cond unit
Twin - Twout
Cooling Tower - Approach
DT leaving and entering
Twout - WBTairin
Cooling Tower - Eff
Eff = R / (R+A)
Partial Pressure
P = Psat @ T * %RH
PSI -> FT head
PSI * 2.31 = FT
Rectangle Equivalent Diameter
Rectangle De = 1.3 (a b)^.625 / (a + b)^.25
ASHRAE 15
Safety for Refrigeration Systems
ASHRAE 34
Classification of Regrigerants
ASHRAE 55
Thermal EV Conditioning for Human Occupancy
ASHRAE 62.1
Ventilation for IA Quality
ASHRAE 62.2
Ventilation for IA Quality - Residential
ASHRAE 90.1
Energy Codes
Natural Gas “name”
Methane
HZ & RPM
1 HZ = 60 RPM
