Industrial Heat Pumps

 

Pinch analysis

The pinch analysis is a tool that can be used to analyze a set of heat flows and to determine whether it is possible to interchange these heat flows. When application of a heat pump is considered, it might be useful to carry out a pinch analysis. The method is particularly useful for complex industrial plants. Goal is to map all heat flows and then connect hot and cold flows that can exchange heat. Furthermore, the analysis shows the amount of cooling and heating needed. A heat pump can be used to couple these needs for cooling and heating.

Procedure

The pinch analysis is a structured method which involves the following steps:

  1. Map all process streams inside and in the vicinity of the plant and compose a mass and energy balance.
  2. Put the different process streams in a table that shows their supply temperature, desired temperature and heat capacity.
  3. Determine the power of the different process streams for different temperature steps and make a graphical representation of these data points.
  4. Find out whether or not it is possible to interchange heat between different process flows with the use of heat exchangers.
  5. Determine the location of the pinch point and find out if more exchange of heat is needed after direct heat exchange is performed.
  6. Depending on the temperature levels and powers a decision can be made on which installation is most suitable to apply.

Example

The different steps of a pinch analyses are explained in more detail in the example that is stated below.

  1. Map all process streams inside and in the vicinity of the industrial plant and compose a mass and energy balance:
    The example is based on four process streams. Two streams that need to be cooled down an two streams that need to be heated.
  2. Put the different process streams in a table that shows their supply temperature, desired temperature and heat capacity:
    Table
  3. Determine the power of the different process streams for different temperature steps and make a graphical representation of these data points. On the x-axis the power (either heating or cooling) is shown and the y-axis shows temperature. Two graphs, one cold stream and one hot stream, are combined in to one graph:
    Pinch graph
  4. By interchanging energy from the cold an hot stream with the use of a heat exchanger, 241 kW of heat can be recovered. Ideally above the pinch point as much a 186 kW of heat is needed while below the pinch point 238 kW of cooling is needed.
  5. The pinch point is determined by the location where temperature differences (delta T) between process streams are the smallest. At this point heat exchangers can operate at minimal delta T. For an increase in delta T at the pinch point due to a change in process temperatures, specifications for heat exchangers change. Through combining the graphs of the two product flows, a joint curve is created that gives insight in process streams and temperature variations. At the pinch point a temperature difference of 0 °C is kept. In practice a temperature difference is needed to be able to have heat exchange.
  6. Depending on the temperature levels and powers a decision can be made on which installation is most suitable to apply: For small temperature difference and a need for heating and cooling that is comparable, a heat pump might be interesting to apply. With the use of a heat pump the cold and hot process streams can be coupled. In this case a heat pump can be installed that has an evaporation temperature of 25 °C and a condensation temperature of 82 °C . The remainder is an additional cooling capacity of 50 kW. For a temperature difference that is too big, efficiency of the heat pump will drop to a non-feasible level in which case alternative heating and cooling system have to be installed. Examples of these alternative installations are cooling towers, refrigeration installations and hot water or steam systems.

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