How to evaluate thermal performance of a vacuum insulated tubing?
k is the thermal conductivity of a material. It is intrinsic and varies with temperature.
- Unit is W/(m.K) or BTU/hr-ft-°F
- The lower thermal conductivity k, the better insulation
- if the system is continuous, divided by the thickness, it gives a first approximate of the U-value.
Objective: compare two different insulation materials
Ex. : compare rockwool insulation and glasswool insulation
How to evaluate thermal performance of a global system?
- U-value is the Overall Heat Transfer Coefficient of a system. Multiplied by the surface reference and the temperature differential between the boundaries it gives the quantity of heat crossing the system. Unit is W/(m².K) or BTU/hr-ft²-°F. It includes the discontinuities of the system. The lower the better.
- K-value is as a thermal conductivity including the discontinuities of the system. Divided by the distance between the boundaries of the system, it gives a good approximate of the U-value. Unit is W/(m.K) or BTU/hr-ft-°F.
Objective: compare overall thermal performance and specifications of two different vacuum insulated tubings.
How to evaluate the heat loss a global system?
Heat loss is the exchanged heat through the boundary of a system.
- Symbol: Φ
- Units: W or Btu/hr
- Formula: Φ = U x Surface x Δ T