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?

Diagram for heat loss

Heat loss is the exchanged heat through the boundary of a system.

• Symbol: Φ
• Units: W or Btu/hr
• Formula: Φ = U x S_urface x Δ T