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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

The overall coefficient of heat transfer U of the wall, floor, or ceiling of a refrigerated space can be derived from:

U = 1 / [ 1 / ( fi + x1 / k1 + x2 / k2 + x3 / k3 + 1 / fo ) ]
Equation 1.0

Where:

U = overall heat transfer coefficient, W/(m2 · K)
x = wall thickness, m
k = thermal conductivity of wall material, W/(m2 · K)
fi = inside film or surface conductance, W/(m2 · K)
fo = outside film or surface conductance, W/(m2 · K)

9.37 W/(m2 · 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/(m2 · 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, m2
Δt = difference between outside air temperature and air temperature of the refrigerated space, °C (to - trs)
to = outside air temperature
trs = 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 Conductivitya k, W/(m ·K) Polyurethane board (R-11 expanded) 0.023 to 0.026 Polyisocyanurate, cellular (R-141b expanded) 0.027 Polystyrene, extruded (R-142b) 0.035 Polystyrene, expanded (R-142b) 0.037 Corkboardb 0.043 Foam glassc 0.044

aValues are for a mean temperature of 24°C, and insulation is aged 180 days.
bSeldom used. Data are only for reference.
cVirtually 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

Related:

Derived from resources provided by:

ASHRAE Pocket Guide for HVAC, 2013