amount of thermal energy exchanged between the system and the environment during the phase transition. The term latent heat is used as the amount of energy (joule unit) and as the amount of energy per unit of substance that changed the state of matter. On our technical sheets, expressed in J/kg.
heat needed to increase body temperature by one unit. The same specific heat can also be defined for cooling. On our technical cards, expressed in kJ/kgK.
Graphite heat exchanger
It is a kind of heat exchanger in which heat exchange takes place inside blocks made of graphite with hollow vertical and horizontal channels in which products flow. Schmidt-Bretten API heat exchangers are characterized by the highest quality of graphite, originating in Japan, which has been impregnated in phenolic resin making it more durable.
the property of fluids and plastic solids characterizing their internal friction resulting from the shifting of fluid layers in relation to each other during the flow. In other words, it is the ratio of shear stress to shear rate. On our technical cards, expressed in the unit cP (centipoise).
Plate heat exchanger
It is a type of exchanger in which heat exchange occurs between vertical plates through channels of which liquids flow or pass through gases. Such an exchanger may have a twisted or welded construction (un-twisted).
the ability of a substance to conduct heat. Under the same conditions, more heat will flow through the substance with greater thermal conductivity. On our technical cards, expressed in the formula W/mK.
Pipe heat exchanger
It is a type of exchanger in which heat exchange occurs between horizontal pipes and the jacket. One product flows through the pipes, the other through the jacket. Such an exchanger has a non-twisted construction.
Heat transfer coefficient
coefficient determining heat transfer through thermal barriers. The higher the better the heat transfer coefficient of the exchanger. On our technical cards it is referred to as K-Value (W/m²K).
Fouling Factors in Heat Exchangers
The fouling factors to be used in the design of heat exchangers are normally specified by the client based on their experience of running the plant or process. If uncontrolled, levels of fouling can negate any benefits produced by careful heat exchanger design. The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning. In reality, if the wrong fouling factor is used, cleaning may actually be required more frequently.
Fouling mechanisms vary with the application but can be broadly classified into four common and readily identifiable types.
Occurs when chemical changes within the fluid cause a fouling layer to be deposited onto the tube surface. A common example of this phenomenon is scaling in a kettle or boiler caused by “hardness” salts depositing onto the heating elements as the solubility of the salts reduce with increasing temperature. This is outside the control of the heat exchanger designer but can be minimised by careful control of the tube wall temperature in contact with the fluid. When this type of fouling occurs it must be removed by either chemical treatment or mechanical descaling processes (wire brushes or even drills to remove the scale or sometimes high-pressure water jets).
Caused by the growth of organisms within the fluid which deposit out onto the surfaces of the heat exchanger. Again this is outside the direct control of the heat exchanger designer, but it can be influenced by the choice of materials as some, notably the non-ferrous brasses, are poisonous to some organisms. When this type of fouling occurs it is normally removed by either chemical treatment or mechanical brushing processes.
Occurs when particles contained within the fluid settle out onto the surface when the fluid velocity falls below a critical level. To a large extent this is within the control of the heat exchanger designer, as the critical velocity for any fluid/particle combination can be calculated to allow a design to be developed with minimum velocity levels higher than the critical level. Mounting the heat exchanger vertically can also minimise the effect as gravity would tend to pull the particles out of the heat exchanger away from the heat transfer surface even at low velocity levels. When this type of fouling occurs it is normally removed by mechanical brushing processes.
Occurs when a layer of corrosion products build up on the surfaces of the tube forming an extra layer of, usually, high thermal resistance material. By careful choice of materials of construction the effects can be minimised as a wide range of corrosion resistant materials based on stainless steel and other nickel-based alloys are now available to the heat exchanger manufacturer.
The use of corrugated tubes has been shown in be beneficial in minimising the effects of at least two of these fouling mechanisms: deposition fouling because of an enhanced level of turbulence generated at lower velocities, and chemical fouling. Chemical fouling is reduced because the enhanced heat transfer coefficients produced by the corrugated tube result in tube wall temperatures closer to the bulk fluid temperature of the working fluids.
Control of dirt and surface
- The basis for deciding how much surface allowance to take on a plate heat exchanger is many years of API and designer experience in various food processes. In some cases it sometimes happens that we also rely on the client’s requirements (experience).
- In justified cases, the API is able to take the risk of estimating how long the lifetime of the exchanger will be extended when designing a specific surplus allowance. However, these are only estimates and this assessment can not be the basis for planning production or washing processes. Precipitation precipitation is a dynamic process, depending on many factors.
- API has experience in working with different products on CD results. Products that pose the greatest challenge in terms of sediments and processes of sedimentation on the plates are juices, dairy products and liquid egg mass and the like.
- Fouling is a computational value Based on this value, the designer of the exchanger using computer software defines the surface allowance during the selection of the exchanger The contamination describes the thickness of the additional layer formed on the heat exchange surface on the exchanger plates.