Thermal Length in Plate Heat Exchangers: Pressing Depth vs. Plate Dimension
•Thermal length is a dimensionless number that allows the design engineer to relate the performance characteristics of a channel geometry to those of a duty requirement.
•Thermal length (Θ) is the relationship between temperature difference DT on one fluid side and LMTD.
•The thermal length of a channel describes the ability of the channel to affect a temperature change based on the log mean temperature difference (LMTD).
•The thermal length of a channel is a function of the channel hydraulic diameter, plate length, and the angle of the corrugations, along with the physical properties of the process fluids and available pressure drop.
•To properly design a PHE, the thermal length required by the duty must be matched with that achievable by the selected channel geometry.
A Plate HX is said to be Optimally Sized, if the thermal length required by the duty can match the characteristic of the channel, by utilizing all the available pressure drop with no over-dimensioning, for any chosen channel geometry.
•To have the most economical and efficient exchanger it is critical to choose, for each fluid, a channel geometry that matches the thermal length requirement of each fluid.
•Since thermal length achievable by a channel depends on the physical properties of the fluid, correction factors must be considered when the fluid’s physical properties differ from those for standard fluid (water)
One criteria to have different thermal lengths is to couple differetn plates corrugation H and L to have Mix channel.
This approach generated so called "Family" series of plates, with common coil width and different heights
But in application as District Cooling size of units if you would like to get one single pass with all connections in front side were going to be huge...
Duty: 2 x 7,555 kW.
water flow rate side 1: 810 m³ / h
water flow rate sidew 2: 720 m³/ h
In/out temp.side 1: 14,30/6,00 °C
In/out temp. side 2: 4,00/13,00 °C
Here below different duties
Thermal length = 5
Thermal length = 10
Manufacturers have developed various characteristic plates in order to accommodate the wide variety of process conditions. The term NTU, or also known as ‘Number of heat transfer units’, indicates high heat transfer effectiveness (large temperature change between the inlet and outlet against very small temperature difference between two flowing mediums). The adoption done to a high-NTU plate is to utilize a pattern with a shallower plate depth and a smaller pitch. This ensures an excellent heat transfer performance.
Variable thermal length
Most of the plate heat exchangers manufactured are available with two different pressing patterns. When the plate has a narrow pattern, the pressure drop is higher and the heat exchanger is more effective. This type of heat exchanger has a long thermal channel. When the plate has a wide pattern, the pressure drop is smaller and the heat transfer coefficient is accordingly somewhat smaller. This type of heat exchanger has a short thermal channel. When two plates of different pressing patterns are placed next to each other, the result is a compromise between long and short channels as well as between pressure drop and effectiveness.
For some duties, cooling applications for example, the temperature program is very tight with close approaches on the different temperatures. This gives what we refer to as high theta duties and requires high theta units. High theta duties are duties that have Θ > 1 and are characterized by:
• Long plate, longer time for the fluid to be cooled
• Low pressing depth that gives less fluid per plate to be cooled