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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 / ( 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:
- Overall Heat Transfer Coefficient Table Charts and Equation
- Combined Overall Heat Transfer Coefficient Equation
- Convective Heat Transfer Coefficient - Heat Transfer
- Convective Heat Transfer Coefficients Table Chart
- Round Duct Heat Transfer Coefficient Wall Temperature Heat Transfer Equation and Calculator
- Overall Heat Transfer Coefficient Table Chart
- Overall Heat Transfer Coefficient Across Heat Exchanger Tubes Equation
Derived from resources provided by:
ASHRAE Pocket Guide for HVAC, 2013
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