## 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?

. 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.**U-value is the Overall Heat Transfer Coefficient of a 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.**K-value is as a thermal conductivity including the discontinuities of the system**

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 S
_{urface}x Δ T