| Getting rid of the heat Tips for the correct use of heat exchangers |
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Cooling is becoming more and more important in the design of hydraulic systems. In selecting a suitable heat exchanger there is however more to be considered than simple performance data. Environmental conditions and often legal regulations are also important. Universal Hydraulik has some useful tips to make the design and selection of a heat exchanger easier.
The rule of thumb is:
Between 10% and 50% of the electrically installed power in a hydraulic system is lost due to friction in pumps, valves and flow regulation units.
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Introduction Some energy is always lost during transfer. In hydraulic systems such losses lead to undesirable heating. This in turn leads to premature ageing of the hydraulic oils used and to increased wear and tear on components. Positioning also becomes less accurate as oil temperature rises, and this can lead to malfunctioning of systems. This means that efficient cooling is just as vital for the good functioning and reliability of a hydraulic system as good filtration.
In selecting a suitable heat exchanger a number of different points must be considered. They include the cooling power required
the cooling medium available
the cooling process used
existing construction parameters
legal regulations.
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Efficient cooling is vital to function It is generally quite a complex task to determine the cooling power required exactly. It is thus usually assumed that between 10% and 50% of the electrically installed power in a hydraulic system is lost through friction in pumps, valves and flow regulation units. This lost power must be removed through the heat exchangers.
If there are other sources of heat energy in the system which could warm the hydraulic oil, these must be taken into account when the required cooling power is calculated.
Water or air is usually available as cooling medium. The choice of medium will depend on local conditions and will have an effect on running costs later.
In the selection process a distinction must be drawn between the cooling of the returning medium (for example oil) and cooling within a separate, secondary circuit. An advantage of cooling in a secondary circuit is that there is always a constant flow of oil through the heat exchanger. This cools the system continuously and avoids dangerous peaks in pressure which could damage the heat exchanger.
Heat exchangers are generally selected with the aid of design software which selects a suitable one from a range of standard sizes on the basis of given operating parameters. Here the heat transmission coefficient, the mean temperature difference and the exchange surface required are particularly important.
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Heat transmission coefficient The heat transmission coefficient measures the transmission of heat between the media in a heat exchanger. It is not only influenced by the heat conductivity and the viscosity of the media, but also by flow conditions. The coefficient thus also depends on the design of the heat exchanger. In simple terms it can be said that the cooling power of the heat exchanger falls with increasing viscosity and improves as the flow becomes more turbulent.
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Mean temperature difference The mean temperature difference is derived from the entry and exit temperatures of the two media in the heat exchanger. Here the cooling power also increases as the temperature difference rises. |
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Mean temperature difference The mean temperature difference is derived from the entry and exit temperatures of the two media in the heat exchanger. Here the cooling power also increases as the temperature difference rises. |
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Exchange surface necessary The exchange surface necessary depends on the cooling power required, the heat transmission coefficient and the mean temperature difference. It thus depends very much on the design of the heat exchanger. Heat exchangers of different designs cannot be meaningfully compared only in terms of exchange surface.
A heat exchanger must satisfy a number of flow, vibration and safety requirements as well as meeting thermo-dynamic criteria. This has led to the development of a number of different designs:
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Tubular heat exchangers The traditional heat exchanger is the tubular type. It consists of a sheath pipe through which the hydraulic oil is conducted and a bundle of thin tubes inside the sheath. The cooling medium flows through this bundle of tubes. Due to the comparatively large gaps between the pipes these heat exchangers only suffer slight pressure losses, even when flow volumes are large or the oils used highly viscous. They are thus particularly suitable for use in lubrication systems.
As the flow in and around the cooling pipes is not generally subject to strong turbulence, the heat transmission coefficient in tubular heat exchangers is relatively low. The exchange surface must thus be correspondingly large for the desired cooling power to be achieved. Cooling units measuring several metres in length are not uncommon.
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Safety heat exchangers The fail-safe principle
Reliable separation of the process and cooling media can be guaranteed by the use of safety heat exchangers.
Illustrations: Universal Hydraulik
Cooling with water always leads to problems when the two media mix following damage to a heat exchanger. In such cases considerable expense is generally incurred due to interruptions to operations and for the repair and maintenance work necessary. If surface waters are contaminated the authorities may also impose large financial penalties and strict environmental requirements.
A reliable separation of the media can be achieved by the use of safety heat exchangers. Here the pipes through which the cooling medium flows are enclosed in additional protective tubes. The gap between the cooling medium and the protective tubes is filled with a barrier fluid designed to ensure good heat transmission.
The pressure in the barrier fluid is monitored constantly so that any leak can be spotted easily. The heat exchanger can thus be replaced in good time before water penetrates the hydraulic system or cooling water is contaminated.
Secondary circuit cooling units represent a further development of oleo-pneumatic coolers. These units can often also be fitted with an additional filter, so that secondary circuit cooling and filtration functions can easily be combined in one piece of apparatus.
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Tubular heat exchangers with lamellas To improve the efficiency of tubular heat exchangers a number of ways have been developed to increase the exchange surface and at the same time reduce the length. For example, there are tubular heat exchangers with thin aluminium lamellas attached to the cooling pipes. This creates narrow gaps in which the turbulence is great, and this improves heat transmission. The aluminium lamellas however increase the loss of pressure in the heat exchanger. They are thus only suitable for moderate levels of viscosity. Due to their compact design they are often used in industrial hydraulic systems. |
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Plate heat exchangers Plate heat exchangers are to be recommended where temperature differences are small. In such cases plate heat exchangers are usually more compact and less expensive then tubular ones.
So-called plate heat exchangers are even more compact then tubular heat exchangers with lamellas. In these the media flow between a number of plates arranged in sequence. These plates are embossed to produce narrow flow channels. Turbulence is thus high and heat transmission good. Plate heat exchangers are always to be recommended where temperature differences are small, as in such cases plate heat exchangers are usually more compact and less expensive then tubular ones.
Different types of plate heat exchanger have gradually been developed. These differ mainly in the way that the individual plates are connected.
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Soldered plate heat exchangers Here the plates are soldered together using copper or nickel. This permits economical series production. The narrow channels between the plates lead to a greater loss of pressure than with tubular heat exchangers. It is thus often necessary where flow volumes are high to use a large number of plates in order to reduce the pressure loss. This in turn partially eliminates the advantage over tubular heat exchangers.
If the cooling water is badly purified the channels between the plates can also easily become blocked. This gradually leads to a drop in cooling performance. Due to their design, it is only possible to clean soldered plate heat exchangers to a limited extent.
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Screwed plate heat exchangers Here the plates hang in a frame linked together by two pressure plates. The advantage of this is that the plates can be removed, enabling the heat exchanger to be cleaned and extended by the addition of further plates if required.
Questions about the cooling medium
Is water available?
How much water can be used for cooling?
How is the water purified?
How high is the surrounding temperature?
What noise restrictions have to be adhered to?
How clean is the environment surrounding the cooler?
A variety of embossings make it possible to optimise both the pressure loss and the heat transmission in screwed plate heat exchangers. Here too however, tubular heat exchangers may be a better choice where temperature differences are great.
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Oleo-pneumatic (oil-air) coolers In selecting an oleo-pneumatic cooler the noise limits imposed under labour laws or building regulations for example must always be observed.
In the heat exchangers described so far water is always used as the cooling medium. Water however is becoming more and more expensive, and air is often used instead, especially when cooling water is not available. In oleo-pneumatic coolers cooling air is sucked through a cooling network using a fan.
Due to its poor thermal properties however, the usefulness of air as a cooling medium is limited. This disadvantage has to be compensated by high flow volumes and a large exchange surface. An oleo-pneumatic cooler thus takes up more space than a tubular or plate heat exchanger of comparable cooling power.
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Oleo-pneumatic coolers are noisier than other types A further disadvantage is the often considerable noise nuisance caused. Noise limits imposed under labour laws or building regulations for example must always be observed.
When an oleo-pneumatic cooler is installed, adequate fresh air ventilation must be ensured. Otherwise the surrounding temperature could rise and the heat exchanger may fail to cool adequately.
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Secondary circuit cooling units Secondary circuit cooling units represent a further development from oleo-pneumatic coolers. Here the motor drives a pump as well as the fan, and this transports the hydraulic oil through the heat exchanger. These units can often also be fitted with an additional filter, so that secondary circuit cooling and filtration functions can easily be combined in one piece of apparatus. The same restrictions as apply to oleo-pneumatic coolers however also apply here. | |  |
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