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Overall Coefficient of Heat Transfer of a Refrigerated Space Formulas and Calculator
Heat Transfer Engineering and Design
Overall coefficient of heat transfer U of the wall, floor, or ceiling of a refrigerated space can be derived from Formula and calculator
Preview: Overall coefficient of heat transfer U calculator
The overall coefficient of heat transfer U of the wall, floor, or ceiling of a refrigerated space can be derived from:
U = 1 / [ 1 / ( f_{i} + x_{1} / k_{1} + x_{2} / k_{2} + x_{3} / k_{3} + 1 / f_{o} ) ]
Equation 1.0
Where:
U = overall heat transfer coefficient, W/(m^{2} · K)
x = wall thickness, m
k = thermal conductivity of wall material, W/(m^{2} · K)
f_{i} = inside film or surface conductance, W/(m^{2} · K)
f_{o} = outside film or surface conductance, W/(m^{2} · K)
9.37 W/(m^{2} · K) for fi and fo is frequently used for still air. If the outer surface is exposed to 24 km/h wind, fo is increased to 34 W/(m^{2} · K).
With thick walls and low conductivity, the resistance x/k makes U so small that l/f has little effect and can be omitted from the calculation.
After establishing U, the heat gain is given by Equation 1.2:
q = U A Δt
Equation 1.2
where
q = heat leakage, W
A = outside area of section, m^{2}
Δt = difference between outside air temperature and air temperature of the refrigerated space, °C (t_{o}  t_{rs})
t_{o} = outside air temperature
t_{rs} = air temperature of the refrigerated space
Latent heat gain due to moisture transmission through walls, floors, and ceilings of modern construction refrigerated facilities is negligible.
Thermal Conductivity of Cold Storage Insulation
[2014R, Ch 24, Tbl 1] 

Insulation 
Thermal Conductivity^{a} k,
W/(m ·K) 
Polyurethane board (R11 expanded) 
0.023 to 0.026

Polyisocyanurate, cellular (R141b expanded) 
0.027

Polystyrene, extruded (R142b) 
0.035

Polystyrene, expanded (R142b) 
0.037

Corkboard^{b} 
0.043

Foam glass^{c} 
0.044

^{a}Values are for a mean temperature of 24°C, and insulation is aged 180 days.
^{b}Seldom used. Data are only for reference.
^{c}Virtually no effects from aging.
Minimum Insulation Thickness
[2014R, Ch 24, Tbl 2] 

Storage
Temperature, °C 
Expanded Polyisocyanurate Thickness


Northern U.S., mm

Southern U.S., mm


10 to 16

50

50

4 to 10

50

50

–4 to 4

50

75

–9 to –4

75

75

–18 to –9

75

100

–26 to –18

100

100

–40 to –26

125

125

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Derived from resources provided by:
ASHRAE Pocket Guide for HVAC, 2013