The ability to transfer heat in liquid and gaseous media depends on the turbulence of the medium. High turbulence is desirable for efficient transfer. Near a plane wall, there is always a film of laminar flow of gas or liquid, depending on the type of medium present (cf. chapter 1.5). The heat transfer in a laminar flow film is poor, because convectional heat transfer is more or less non-existent. However, if higher turbulence can be achieved, the insulating film becomes thinner, convection increases and, consequently, heat can be transported more efficiently, as shown in Figure 1.3.
The Reynolds (Re) number is a dimensionless number used to describe the "state" of a fluid (turbulent or laminar flow). The Reynolds number is defined as in equation 1:
(For SWEP's heat exchangers, the hydraulic diameter is approximated as twice the pressing depth for the brazed plate heat exchanger.)
High turbulence is achieved by increasing the disturbances in a flow. A rough surface thus results in a more turbulent flow than a plane surface.
The flow inside a brazed plate heat exchanger is much more turbulent than the flow inside a Shell & Tube (S&T) heat exchanger, for example. This is because the plates of the brazed plate heat exchanger are rough and folded due to the herringbone pattern (see Figure 1.13). By contrast, in an S&T the fluids flow through flat pipes. Full turbulence is reached at approximately Re = 2300 in a tube but at an Re as low as approximately 150 in a brazed plate heat exchanger, which indicates that a smaller flow velocity is needed in a brazed plate heat exchanger than in an S&T.
For a proper comparison between the Reynolds numbers of different passages, their geometries should be exactly the same. This is of course not the case for a comparison between a brazed plate heat exchanger channel and an S&T channel, which have different hydraulic diameters. However, the practical fact remains: inside a brazed plate heat exchanger, a lower flow velocity is required to achieve fully turbulent flow.
In addition to the herringbone pattern on the plates, other parameters that lead to a high turbulence include:
- Small cross-sectional channel area
- Low fluid viscosity
With a constant pump power, a smaller cross-sectional channel area leads to a higher velocity and better heat transfer. For brazed plate heat exchangers, a smaller cross-sectional area is obtained if a narrower plate is chosen, while lower velocity is obtained when more plates are added. The "cost" of high velocity and small cross-sectional area is an increased pressure drop through the heat exchanger. When designing a heat exchanger, e.g. in SWEP Software Package (SSP), the heat exchanger with the calculated pressure drop closest to the maximum allowed should be selected, in order to achieve maximum efficiency.
Viscosity is also an important factor when discussing flow regimes. For example, oil has a higher viscosity than water, and it is therefore more difficult to achieve turbulence in oil flows. A medium with a low viscosity might therefore be more useful as a heat conductor.