In a typical developed country as much as 40% of total fuel consumption is used for industrial and domestic Space Heating and process heating. Of this, around one third is wasted. This wasted heat can be lost to the atmosphere at all stages of the process; through inefficient generation, transmission or final use of that energy. Waste heat recovery aims to minimize the amount of heat wasted in this way by reusing it in either the same or a different process.
Waste heat can be recovered either directly (without using a heat exchanger—e.g., recirculation) or, more commonly, indirectly (via a Heat Exchanger). Direct heat recovery is often the cheaper option but its use is restricted by location and contamination considerations. In indirect heat recovery, the two fluid streams are separated by a heat transfer surface, which can be categorized as either a passive or active heat exchanger. Passive heat exchangers require no external energy input (e.g., Shell and Tube, Plate, etc.) whilst active heat exchangers do (e.g., thermal wheel, Heat Pump, etc.).
When considering waste heat recovery, the key question is always that of financial justification: "How much money will be saved?" The decision to recover waste heat depends critically on whether the resulting energy cost savings outweigh the installed cost of the proposed waste heat recovery project. As a general rule of thumb, a waste heat recovery project is unlikely to be installed if its payback period is longer than two or three years.
When describing waste heat recovery it is important to specify the nature of the waste heat in terms of temperature and material phase. Waste heat can be considered as either low grade (<100°C), medium grade (100°C–400°C) or high grade (>400°C). Low grade waste heat can only be recovered effectively when there is a high quantity of waste heat and a ready use for it. There are many examples of successful heat recovery projects for temperatures between 100°C and 200°C. At 200°C and above most users should be able to make significant cost savings from heat recovery. Different techniques and heat exchanger materials are used to recover heat above 400°C.
It can be difficult to recover heat from solid material and therefore most heat is recovered from and passed to material in a gas or liquid phase. The choice of heat exchanger depends critically on both the temperature and material phase.
Many relatively simple processes have a surprisingly large number of potential waste heat sources and sinks. It is often the identification of the sources and sinks and their relative suitability and proximity that determines the cost-effectiveness of any heat recovery project. Some common sources and sinks for processes, utilities and buildings are listed in Table 1.
In general, high grade waste heat is mainly limited to the iron and steel, glass, nonferrous metals, bricks and ceramics industries. Medium grade waste heat is most widely found in the chemicals, food and drink, and other process industries, as well as building utilities. Low grade waste heat can be found in virtually all areas of industry and buildings and is often the hardest to recover cost-effectively—typical examples of recovering low grade waste heat would be ventilation or hot water systems.
How do you set about recovering waste heat? Waste heat recovery does not always require high capital investment and in some cases little or no cost is involved. The first and often easiest step is to ensure that the heat is not wasted in the first place. This includes:
ensuring plant is operating at maximum efficiency;
reducing evaporation and heat loss from open tanks;
optimizing the scheduling and control of operations;
making sure there are no leaks in ducts and pipes;
fitting insulation and ensuring that it is replaced after maintenance.
Good housekeeping is essential before even considering any major capital investment in waste heat recovery. If wasted heat can be limited at source, then recovering it is made that much easier.
If, after such measures have been taken, there is still heat either exhausting to the atmosphere or draining away, under what circumstances is it worth recovering some of it? Any answer to that question, of course, depends very much on the individual circumstances of each site, in particular whether there is a use for any heat recovered. To assess the suitability of any stream for use in a heat recovery system, the following key parameters must be taken into account:
How much heat is available? This can be derived from plant descriptions or by direct measurement. The amount of available heat can be calculated from the following basic energy flow rate equation:
= Energy flow rate (kW)
S = Cross sectional area (m2)
V = Flow velocity (m/s)
ρ = Fluid density (kg/m3)
cp = Fluid specific heat capacity (kJ/kg K)
ΔT = Temperature difference between heat source and heat sink.
Is the recovered heat at a sufficiently high temperature to be useful? If not, would a heat pump be appropriate?
What are the requirements for the quantity of heat recovered? Can the recovered heat be put to good use?
How much will it cost to transfer heat from the source to the sink? Would a run-around-coil be more cost-effective? Does the heat source stream contain any contaminants?
Do the patterns of heat availability (at source) and use (at sink) coincide, in terms of both quantity and timing? Is the cost or option of storage acceptable?
Having identified all the potential heat sources and sinks and considered the above parameters for each stream, the next stage is to specify what combination of sources and sinks could cost-effectively be combined. For simple applications with only a few obvious sources of waste heat (e.g., an industrial dryer), this process can be done through basic common sense. However, where a large number of potential heat sources and sinks exist (e.g., a typical chemical site), a more systematic analysis of the options is required. Process Integration is a tool to assist in such an analysis.
Put very simply, process integration can be split into 5 basic steps:
By carrying out a site survey, derive the temperatures and heat capacity flow rates for each stream and specify whether a stream is a heat source or heat sink.
The stream data sets are then combined to produce a hot and cold composite curve for that particular process as shown below (Figure 1). This is a plot of the temperature against enthalpy for that process and shows the amount of heat available and required at various temperatures.
By combining these two curves onto a single diagram and moving the position of the curves to the point of closest approach (or "Pinch" point) represented by the minimum approach temperature of the heat exchanger, the diagram immediately shows the maximum scope for heat recovery within that particular process (Figure 2).
At this point in the analysis it is possible to consider the impact of changing process conditions to increase the amount of waste heat recovered. This iterative stage can be very important.
The final stage is to design the actual heat recovery network to achieve the minimum heating requirement identified in step 3. This network is derived using a few basic rules of process integration, usually via software. The designer can then carry out a cost/benefit analysis on all recommended heat exchangers to produce the most cost-effective final heat recovery network.
Once the hot and cold streams have been identified, suitable heat exchangers must be selected. By far the most commonly used type of heat exchanger is the shell and tube, although a wide variety of other types are also used. A few of the more common heat exchangers are listed in Table 2.
These heat exchangers are mainly used to recover medium and low grade heat. To recover high grade heat (i.e., above 400°C; usually from gases) the following equipment is commonly used: regenerator beds; tube or compact recuperators; self-recuperative burners; regenerative ceramic burners, waste heat boilers.
Typical efficiencies for different types of heat exchangers vary from around 90% for plate and other compact heat exchangers to around 70% for shell and tubes. The efficiency of a heat exchanger can be expressed in terms of temperature as follows:
where: hot stream enters the heat exchanger at T1 and leaves at T2 and cold stream enters the heat exchanger at T3 and leaves at T4. The efficiency of a heat exchanger is greater when the two streams are arranged in counter rather than parallel flow. Other factors to consider when specifying a suitable heat exchanger are: the Fouling characteristics of the two streams; the potential for recovering sensible and latent heat (and the possible Corrosion implications of the latter); and the cost of both the heat exchanger itself and the ductwork, etc. necessary for its installation (N.B. the installed cost of a heat exchanger is often around four times the capital cost).
Heat & Mass Transfer, and Fluids Engineering