Heat Transfer in Plate Heat Exchangers
Heat transfer is the core function of a heat exchanger.
The laws of thermodynamics provide the basic rules for heat-exchanger design:
1st Law of Thermodynamics: A heat exchanger cannot destroy or create energy, but there is always a balance between the hot and cold sides.
2nd Law of Thermodynamics: Heat flows from a hot medium to a cold medium to equalise the temperature difference.
Heat transfer is the exchange of heat between hot and cold bodies. There are three fundamental modes by which this occurs:
Heat transfer in a heat exchanger is a combination of conduction and convection. Conduction and convection are calculated in each heat-transfer case.
Calculating heat transfer
The total heat-transfer coefficient U is calculated by the sum of the convective heat transfer from both sides (hhot and hcold) of the heat-transfer surface and conduction (k) through the heat-transfer plate.
Conductivity is calculated using the thermal conductivity of the plate material and the material thickness. The plate material is typically stainless steel with thicknesses ranging from 0.7 mm to 1.5 mm. The thinner the material is, the more efficient the conduction through it will be. However, the material conductivity of plates up to 1.0 mm is typically insignificant when compared to the total heat transfer.
More important than conduction is the convective heat transfer between the plate surface and flowing medium. Convection, or convective heat transfer, is the transfer of heat from one place to another through the movement of fluids (h in the above formula).
Convection is the most challenging property to calculate. There are different calculation methods for different geometries, as well as for liquids, gases, condensing and evaporating heat-transfer cases. Theoretical equations, such as Dittus-Boelter, are then verified and adapted through experimental tests on different heat-exchanger types.
Convection is the dominant mechanism in plate heat exchangers, thus it is important to improve the convective forces.
The convective heat transfer’s coefficient is affected by the fluid properties and the geometry of the heat-exchanger surface. A given fluid is typically defined by the process and cannot be changed, but the heat-exchanger type and structure can be engineered to find the best heat transfer.
Why heat transfer is better in Vahterus Plate & Shell Heat Exchanger vs. Shell & Tube heat exchangers
The use of heat-transfer plates, rather than tubes, increases the surface area available to the flow, as well as creating a flow path that induces greater turbulence at lower flow velocities. The flow between the plates has a higher rate of boundary layer reattachments when compared to pipe flow, which explains the difference in heat-transfer coefficiency. Boundary layer reattachment means that the fluid is not flowing smoothly over the heat-transfer surfaces, and the flow direction changes constantly even at low velocities, which subsequently enhances heat transfer.
Read case studies where Vahterus Plate & Shell Heat Exchangers have been used to replace old tube heat exchangers and gasket heat exchangers in order to improve a system’s reliability and make it easier to maintain.