Waste heat recovery is not only done with a heat exchanger. Having an efficient heat recovery system requires looking into heat transfer fluids, design parameters, instrumentation and piping. Considering these aspects can give you the decisive advantage for profitabilty, durability and safety.
The idea of heat recovery is to extract as much energy as possible from a waste heat source and make it usable for any application, where it can replace fossil energy.
For the optimum benefit, it makes sense to cool down the waste heat potential as much as possible. But, there are two conditions to be able to do that. Firstly, the acid dew points, since the condensate of waste gas can cause corrosion. As an approximation, the acid dew points range from about 120 °C to about 150 °C (250 to 300 °F). The second limit is the temperature of the application (consumer) at which the heat should be used. The first condition can be compensated by using materials, that can withstand the corrosion. For the second condition, it is important to use a heat carrier suitable for the individual application.
Choosing the heat carrier depends mainly on the temperatures of the heat consumer.
As heat carriers liquids, vapours or gases can be used. Depending on the technical and commercial condition, all heat carriers offer their individual advantages and disadvantages. Depending on the type of heat transfer media and individual properties, they all have special requirements that must be taken into consideration when selecting a heat carrier.
Due to its excellent properties for heat transfer at common temperatures, we start with heat recovery systems, using water as heat carrier.
Water in liquid condition is an excellent heat carrier medium, since its specific heat capacity is high and the cost is low. With this being stated, its usability is limited in terms of temperature. First off, there is the fact that it freezes under normal conditions at around 0°C (Ice cannot be pumped into pipelines and therefore cannot be used as a heat carrier). The water can be mixed with anti-freezing agent, such as alcohol or glycol, to extend the usable temperature range of water for low temperatures where environmental conditions can lead to freezing.
Next off, as soon as the temperature rises above 100°C, the water boils under normal conditions. Here it must be pressurized to remain liquid. The higher the temperature, the higher the pressure will be. Higher temperatures / Pressures require more effort to pressurize the system, resulting in higher investment and maintenance cost.
Pressurization is commonly achieved by installing an expansion tank with a membrane, where the water is on one side of the membrane and on the other side is a compressible medium such as Nitrogen, which has a certain pressure. If the size of the membrane expansion tank is well calculated and the pressure is correct, the unit can maintain the pressure in the system at a constant level. For larger systems or higher temperatures, the pressurization can be achieved with pumps to keep the pressure in the system stable.
At a certain temperature, the cost for these measures overweigh the benefits of the good physical properties. Under normal conditions, at temperatures above 120°C, organic heat carrier fluids are more suitable due to its ability to handle high temperatures without steam pressure.
Generally all parts of the system must withstand the system pressure, which is limited by the safety valve. This means, if the safety valve opens at 10bar, all parts of the system must be designed for an operation pressure of at least 10bar. The same applies to the temperature, all parts of the system must withstand at least the temperature, where the safety temperature limiter switches off the heat supply.
Plants with liquid heat carriers, generally should be equipped with flow monitoring devices and safety temperature limiters.
All parts, where heat is supplied into the system, must be secured with a suitable safety valve. The safety valve will open and release the pressure if the temperature controller and safety temperature limiter is not working, e.g. if there is a failure in the temperature control system.
Besides, it is very reasonable to have valves at the in- and outlet side of each heat exchanger. In addition to that, a discharge valve is recommended. In this way it is possible to separate every part from the rest of the system and to discharge it in case of a requirement for maintenance, repair works, etc.
It is advisable to have a pressure gauge at in- and outlet of each heat exchanger. Especially in larger systems, where many heat exchangers are combined, it is necessary to have the pressure indications in order to adjust the system in a way that each apparatus is supplied with the suitable heat transfer medium flow. In order to be able to adjust the flow in the system, at each consumer, the valve at the in- or outlet side should be carried out as throttle valve to adjust the pressure drop and therefore the flow in the relevant area.
Also a temperature indication is advised at any in- and outlet connection of the heat exchangers. In this way it is possible, to control the heat transfer at any time and to find out early enough, if the capacity / efficiency goes down, for example due to deposits on the heating surface.
For the heat transport, liquid heat carriers generally require a pump in order to circulate the liquid within the system so the fluid takes the heat in the heat recovery unit and releases it to the heat consumer.
Wherever a pump is installed, it should be protected by a filter at the inlet side. Furthermore, a valve should be installed at the in- and outlet side of the pump to shut off the pump in case of a repair or if the filter needs to be cleaned.
To check the condition of the pump and the filter, a pressure-vacuum gauge should be installed on the suction- and pressure side the pump.
If the temperature at the consumer must be controlled, a temperature control device is required. For example, the temperature can be controlled by a bypass, which leads the hot source completely or partly in bypass to the heat recovery unit.
An expansion system which compensates the temperature related changes of the density of the heat carrier is generally needed in systems, where incompressible heat transfer media is used. This expansion system usually also provides necessary static pressure at the inlet of the circulation pumps.
The pipes of a hot water system can be made of suitable mild steel, copper, plastic etc., since it is a closed system without oxygen access, it requires no corrosion protection from inside. The material and the wall thickness of the pipes must be calculated in accordance to the pressure (remember, the pressure depends on the temperature). The diameter of the pipes depend on the volume flow of the water causing the velocity. If the velocity of the water in the pipes becomes too high, the resistance increases and leads to a higher energy consumption of the circulation pump. For hot water systems, a velocity of 2m/s or below is usually efficient. Please note, the increase of the pressure loss compared to the increase of the volume flow is a square function, this means that a doubling of the volume flow causes a 4-fold higher pressure loss.
For warm- or hot water systems the common connections for armatures, heat exchangers, tanks or other equipment can be flanges, thread connections, cutting ring connections etc. To connect the pipes within each other, there are lots of systems available, like; welding, thread connections, soldering, press connections or even gluing.
Since the entire system is designed to safe thermal energy, reducing heat losses as much as possible by thermal insulation is necessary. The materials and thickness of the insulation depends on the temperatures of the surface, as well as on the surrounding temperature and the conditions, such as out- or indoor installation etc. Therefore, it must be calculated individually for each project.
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