Frequently Asked Questions

1. What is Pinch Analysis?
2. Who developed Pinch Analysis?
3. Are there any uses for Pinch Analysis other than heat integration?
4. Can you explain some of the terminology being used by this application?
5. Who wrote this online application?
6. Why is it bad to transfer heat across the pinch?
7. Is it possible to have a process without a pinch point?
8. What constraints are there on the selection of a feasible stream matching?
9. What are some common values for DTmin?
10. What are some limitations of this online application?

1. What is Pinch Analysis?
   
   • Pinch Analysis (also known as process integration, heat integration, energy integration, or pinch technology) is method for minimising the energy costs of a chemical process by reusing the heat energy in the process streams rather than outside utilities.
     
     The process requires three pieces of data from each process stream: the heat load (enthalpy) in kW or Btu, the source temperature in °C or °F, and the target temperature in °C or °F. The data from all streams are combined in order to create plots of enthalpy against temperature, called composite curves. Four composite curves are needed, curves for the hot and cold process streams, a combined plot of both the hot and cold composite curves, and the grand composite curve. From the combined curve plot we can see the region where the distance between the hot and cold curves is at a minimum, this region is called the pinch point. This provides an important constaint for the design of our heat exchanger network, it is only after this constraint is determined that we can design a heat exchanger network that can meet our ideal minimum energy requirements.
     
     By adjusting the minimum approach distance, known as DTmin, we can find a balance between the ideal minimum utility requirements and the total surface area of our heat exchanger network. As the value of DTmin is made smaller our utility needs go down, but at the same time the required total surface area of our heat exchanger network is increased in order to achieve the necessary amount of heat transfer between our process streams. Therefore, DTmin indicates a bottleneck in the heat integration system.
     
     From the combined composite curve and grand composite curve plots we can easily see the temperature that the pinch point falls on, as well as the ideal minimum heating and cooling utility requirements. It is very important to know the pinch temperature in order to maximize the process-to-process heat recovery and minimize the utility requirements as process-to-process heat exchange should not be occuring across the pinch. This constraint is applied automatically in this program, allowing the user to only place heat exchangers between process streams on only one side of the pinch or the other, but not on both sides at the same time.
   
   
2. Who developed Pinch Analysis?
   
   • Pinch Analysis was originally developed by Bodo Linnhoff and John Flower in 1978 at the University of Leeds. The Problem Table Algorithm that they developed is heavily used by this web application.


3. Are there any uses for Pinch Analysis other than heat integration?
   
   • Over the years the concept has been extended beyond heat integration. Variants of it include mass exchange networks (El-Halwagi and Manousiouthakis, 1989), water pinch (Wang and Smith, 1994; Hallale, 2002; Prakash and Shenoy, 2005), and hydrogen pinch (Hallale et al., 2003; Agrawal and Shenoy, 2006).


4. Can you explain some of the terminology being used by this application?
   
   • A few of the terms being used in this web application may not be entirely intuitive.
     • Source Temperature [TS] - The temperature that a process stream is available at, before any heating or cooling is performed.
     • Target Temperature [TT] - The temperature that you would like the process stream to be heated or cooled to (ie. preheating a reactor feed.)
     • Thermal Capacity [MCp] - Also sometimes called the heat capacity flow rate, this is the mass flow rate of the stream multiplied by its specific heat capacity.
     • Heat Load [dH] - The amount of enthalpy change of the process stream and therefore the maximum amount of heat you could transfer to or from the stream.
       [Q = dH = MCp x (TS - TT)]
     • Composite Curve - A set of individual temperature vs enthalpy curves combined to create a single curve. The composite curve gives a visual profile of the availablity of heating or cooling available from the process streams.
     • DTmin - On the combined composite curve, this is the minimum approach distance between the hot and cold composite curves on the temperature axis.
     • Pinch Temperature - The temperature at which the ability to transfer heat between the process streams is most constrained. Pinch temperature is affected by the chosen value of DTmin and is easily found on the grand composite curve as the temperature at which enthalpy equals zero.
     • Ideal Minimum Heating - The ideal minimum amount of heating utility required to make up for that cannot be performed via the process streams. Can be read from either the combined or grand composite curve plots by looking above the pinch temperature.
     • Ideal Minimum Cooling - The ideal minimum amount of cooling utility required to make up for that cannot be performed via the process streams. Can be read from either the combined or grand composite curve plots by looking below the pinch temperature.
     • Combined Composite Curve - A plot of both the hot and cold composite curves. DTmin is the closest distance that they can approach on the temperature axis. The areas where the two curves do not overlap show the minimum utility requirements by reading the ethalpy axis. This plot easily illustrates the effect that the chosen value of DTmin has on the minimum heating and cooling utility requirments.
     • Grand Composite Curve - A plot of the overall variation of heat supply and demand across the entire process. The pinch temperature, minimum heating, and minimum cooling utilities are more easily found by inspecting this plot.
     • Stream Splitting - In the case that there are not enough hot streams to match with cold streams (or vice versa) there is the option to split a stream into multiple streams. Each substream will start with the same source and target temperature, but the flow rates and therefore the thermal capacities will be different, giving a wider set of heat loads to work with when matching streams.
     • Stream Matching - When a pair of hot and cold process streams are matched a heat exchanger is created to handle the transfer of heat from the hot stream to the cold stream. The web application determines the amount of heat that can be transfered between the two streams, what the new temperatures of the streams will be at their respective heat exchanger outlets, and the log mean temperature difference (LMTD).
     • Temperature Interval Diagram - A visual representation of the hot and cold process streams, showing their relationship in regards to the pinch point and the amount of heat that can be transferred to other streams. Red vertical lines indicate hot streams, blue vertical lines indicate cold streams, and black horizontal lines indicate the temperature intervals with the thickest black line indicating the pinch temperature. This diagram aids the user in determining how they may transfer heat from hot streams to cold streams without crossing the pinch.
     • Stream Matching Diagram - A visual representation of all streams and their matching relationships. The matched streams are color-coded to make them easier to distinguish. Any left-over heat in a stream is indicated by a light brown color indicating that a utility is needed to heat or cool it.
     • Utility - A source of heating or cooling that does not come from a process stream. The objective of pinch analysis is to minimize the need for utilities.


5. Who wrote this online application?
   
   • The Online Pinch Analysis Tool was written in spring of 2010 by Jeffrey S. Umbach as an undergraduate research project while attending the University of Illinois at Chicago. More information can be found here.


6. Why is it bad to transfer heat across the pinch?
   
   • The combined composite curve shows us a temperature gradient with the heating utility at the top and the cooling utility at the bottom. The second law of thermodynamics tells us that thermal energy can be transferred up a temperature gradient only if work is being done on the system, such as with a heat pump. Therefore if we are trying to perform all of the heat transfer between streams with only heat exchangers, which do no work, then thermal energy can only cascade down the temperature gradient. Because of this we cannot transfer excess thermal energy up to a higher temperature interval.
     
     The pinch temperature marks a point where the thermal energy cannot cross. The energy cascade can occur down the gradient above and below the pinch, but not across it. Above the pinch there is a shortfall of energy, which must be supplied by a heating utility, below the pinch there is an excess of energy which must be rejected to a cooling utility. Likewise all cooling above the pinch and all heating below the pinch must be performed by transferring heat between the process streams. Attempting transfer energy across the pinch is wasteful as the additional heating utility needed above the pinch will only be increasing the need for cooling utility below the pinch. Therefore to minimize both utility needs it is essential to only perform heat transfer between streams on the same side of the pinch.


7. Is it possible to have a process without a pinch point?
   
   • Yes, although such occurences are not common. These are referred to as threshold problems and only feature a heating or cooling utility, but not both.


8. What constraints are there on the selection of a feasible stream matching?
   
   • When the user selects a stream that is to be heated or cooled, certain criteria must be met when the application searchs for a suitable stream to be matched to it. This criteria differs somewhat depending on whether the streams are above or below the pinch.
     • Above the Pinch:
       • All hot streams that contact the pinch must be cooled before hot streams that do not contact the pinch may be cooled.
       • When matching streams at the pinch, the thermal capacity of the hot stream must be less than or equal to thermal capacity of the cold stream.
     • Below the Pinch:
       • All cold streams that contact the pinch must be warmed before cold streams that do not contact the pinch may be warmed.
       • When matching streams at the pinch, the thermal capacity of the hot stream must be greater than or equal to thermal capacity of the cold stream.
     In either case, at any point inside the heat exchanger the difference of the temperatures of the hot and cold streams must be greater than or equal to the value of DTmin.
     [(Tho - Tci) >= DTmin and (Thi - Tco) >= DTmin must both be true.]


9. What are some common values for DTmin?
   
   • Some typical values for DTmin are:
     • Oil Refining - 20-40°C
     • Petrochemical - 10-20°C
     • Chemical - 10-20°C
     • Low Temperature Processes - 3-5°C


10. What are some limitations of this online application?
    
    • This program covers many of the basics of pinch analysis, but there are a few items outside of its scope. The following steps are not performed by this application.
      • Estimation of the number of heat exchangers required.
      • Estimation of heat exchanger surface area (requires heat transfer coefficients.)
      • Estimation of utility costs.
      • Estimation of capital costs.
      • Estimation of optimal DTmin (requires capital costs estimate.)
      Such tasks would have to be performed by the user themself.

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"Online Pinch Analysis Tool" ©2010, Jeffrey S. Umbach. (Written in PHP 5) Based on "Pinch Analysis" ©2007-2010, Ludwig C. Nitsche. (Written in Fortran 95) This program is free for personal and educational use.