Case Study # 1
Heat Recovery System
Measurements
The first step in this case study was the measurement of the conditions of
the exhaust gas of the generator. In order to properly design a heat recovery
system , the following conditions had to be defined:
- Profile of the load along the day (since the variation of temperature of
the exhaust gas depends on the variations of the load of the generator);
- Temperature of the exhaust gas;
- Composition of the exhaust gas;
- Flow rate of the exhaust gas.
Some of the measurements had technical and practical difficulties. During
the project, all alternatives were explored to overcome these difficulties
and after investigating all of them, educated assumptions were done.
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Electrical load and temperature
The temperature of the exhaust gas varies with the load. The higher is the load, the higher is the temperature. Variations of load along the day were estimated, and then measurements of temperatures against electrical load were taken. Care must be taken during this kind of measurement due to the high temperatures involved. Special protection must be worn and close exposition to the exhaust gas must be avoided. Using an appropriate digital thermometer, with a maximum admissible operational temperature of 700°C, a curve of temperature against load was plotted. The minimum temperature of the fumes was measured without any load, which was 103°C. Considering the maximum electrical needs of the farm, this temperature reaches 390°C. However, according to the manufacturer's specification for the generator at the farm, the maximum temperature of the exhaust gas is 540°C; indeed, the farm does not use the maximal load deliverable by the generators.
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Temperature along the day
Matching the previous curve with the typical profile of the load, the variation of the temperature of the exhaust gas along the day was determined and plotted in the following graph:
It can be seen that the maximum temperature is delivered between noon and 14:00. Indeed, this matches with the use of all the equipments in the kitchen (i.e., dish washers, ovens, lighting).
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Composition
The definition of the composition of the exhaust gas is very important, considering that it is related to the main properties of the exhaust gas. Portable exhaust gas measuring and monitoring systems are available in the market, but their high prices limited the use in all applications.
In this way, an approximate calculation of the exhaust gas composition was performed considering typical values found in literature, the combustion reaction, and the specification of operational conditions provided by the manufacturer of the generator.
Taking into account the purpose of the experiment, the accuracy of this assumption was considered acceptable. -
Flow rate
The measurement of flow rate of the exhaust gas was one of the most challenging steps on the experimental procedure because of several factors:
The high temperatures of the exhaust gas make the use of simple anemometers impossible. Equipment like these can be severely damaged in seconds.
Similarly, the use of a pitot tube is not reliable. The pulsating and turbulent nature of the fluid impedes reliable and stable measures.
Recently, an apparatus that applies a correction factor for the differential pressure measured overcoming the pulsating variation of the fluid was developed but this piece of equipment was not available for the experiment.
This kind of diesel generators have diffuse air inlet, which makes it difficult to even a balance of mass in the generator.
Therefore, for all calculations the flow rate indicated in the manufacturer’s specification was assumed (17.4 m3/min) , ignoring the variation across different loads.
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