Liquid heat exchanger equations
A double seal forms pockets open to atmosphere to prevent mixing of product and service liquids in the rare event of leakage past a gasket. Recent developments have introduced the double wall plate. The plates are grouped into passes with each fluid being directed evenly between the paralleled passages in each pass. An important, exclusive feature of the plate heat exchanger is that by the use of special connector plates it is possible to provide connections for alternative fluids so that a number of duties can be done in the same frame [Lane , Hargis et al.
Plates are made from a range of materials, for example, the "Paraflow" plates are pressed from stainless steel, titanium, Hastelloy, Avesta SMO, Avesta SLX or any material ductile enough to be formed into a pressing. The special design of the trough pattern strengthens the plates, increases the effective heat transfer area and produces turbulence in the liquid flow between plates.
Plates are pressed in materials between 0. Up to plates can be contained within the frame of the largest Paraflow exchanger, providing over m 2 of surface area. Flow ports and associated pipework are sized in proportion to the plate area and control the maximum liquid throughput. The temperatures shown are maximum, therefore possible simultaneous chemical action must be taken into account when selecting the most suitable material for a particular application.
Plate performance is determined by the plate geometry but it is not possible to estimate the film coefficient from the trough dimensions with some accuracy as can be obtained with a tube. The geometrical parameters involved such as plate gap, height, pitch and angle of the trough are too numerous for this to be possible but some work has been done on evaluating the effect of these variables [Kays and London ; Maslov ]. In a plate heat exchanger, the heat transfer can best be described by a Dittus-Boelter type equation:.
Typical velocities in plate heat exchangers for waterlike fluids in turbulent flow are 0. All heat transfer and pressure drop relationships are based on either a velocity calculated from the average plate gap or on the flow rate per passage. The film coefficients are very high and can be obtained for a moderate pressure drop.
One particularly important feature of the plate heat exchanger is that the turbulence induced by the troughs reduces the Reynolds number at which the flow becomes laminar. Typical values at which the flow becomes laminar varies from about to , according to the type of plate. The friction factor is correlated with:.
In many applications, the heat transfer surface of the plate is less susceptible to fouling than a tubular unit. This is due to 4 principle advantages of the plate design:. There is a high degree of turbulence, which increases the rate of foulant removal and results in a lower asymptotic value of fouling resistance. The velocity profile across a plate is good. There are no zones of low velocity compared with certain areas on the shell side of tubular exchangers.
Corrosion is maintained at an absolute minimum by careful selection of use of corrosive resistant materials. The most important of these is turbulence. HTRI Heat Trasfer Research Incorporated has shown that for tubular heat exchangers, fouling is a function of low velocities and friction factor.
The definition of LMTD is shown in equation Please note that the logarithmic mean temperature difference LMTD may be used only for single-phase calculations see chapter 6. Note that equation 5 is valid only for one-phase heat exchange.
The specific heat capacity varies for different liquids and different temperatures. Equations 2 and 5 together describe the preservation of energy inside a BPHE, which is shown in equation 6.
This equation, as well as Figure 1. Navigation Refrigerant handbook 1. Basic heat transfer 1. Area A Increasing the area of a heat exchanger implies that more energy can be transferred. Heat transfer coefficient k Energy may be transported from a hot fluid to a colder fluid in three ways: Conduction — The heat is conducted through solid material or a stationary liquid.
In the stainless steel walls of a heat exchanger and in laminar flow slow moving regions, heat is transported only by conduction. The conductivity varies with the physical properties of the medium. Increasing the area of a heat exchanger implies that more energy can be transferred. Hence, increasing the surface area implies higher costs. In BPHEs, the energy is therefore transferred through conduction and convection, and examples of these types of energy transport are shown in In BPHEs, the energy is therefore transferred through conduction and convection, and examples of these types of energy transport are shown in Figure 1.
The space between the dotted lines and the wall in Figure 1. The heat transfer rate within the film is significantly lower than in the bulk liquid, because the temperature gradient decreases dramatically in this area see Figure 1. The reason for the poorer heat transfer is the laminar flow that is always obtained near a plane wall. Laminar flow does not transfer energy as well as turbulent flow. The overall heat transfer coefficient k describes the total effect of conduction and convection on the energy transfer:.
The essence of equation 3 is that a high film coefficient and thermal conductivity and a thin plate lead to a high k-value. Thermal conductivity is a material-specific constant, and the film coefficient is a measure of how well heat is transferred by a specific fluid.
With a higher overall heat transfer coefficient k , more energy can be transferred per heat transfer area. Because this leads to a more costeffective heat exchanger, it is very important to improve the k-value by all means possible.
The temperature difference between the hot and cold media is the driving force in energy transfer.