Theory Flashcards
Explain the steps required to construct balanced composite curves:
- Define process stream system to be analyzed
- Select streams and collect stream data (F*Cp, Tstart, Ttarget)
- Decide which streams are hot (streams that needs cooling) and what streams are cold (streams that need heating)
- Draw the streams and find all temperature intervals and combined F*Cp for each interval.
- Construct composite curves (Q, T) for hot and cold streams seperately (dQ = F*Cp *dT)
- Identify dTmin and create the Balanced Composite Curve (BCC)
- Identify hot and cold utility demand Qh,min and Qc,min.
- Compare available hot and cold utilities regarding temp range, cost etc. Find the mix that optimizes cost.
Extra: Hot utility is usually more expensive than cold. If possible use excess heat or Low Pressure and low temperature hot utility, cheapest. If possible use river cooling or similar for cold utility, or other cooling water.
How can the results from a Balanced Composite Curve (BCC) be used?
The main use of the BBC is (area targeting) to estimate the heat exchanger area required to achive MER.
- Identify cooling and heating demand
- Types and mix of utilities that can be used
- Required heat transfer area
- Investment and operating cost
- How the system wil change if dTmin changes
Explain the steps required to construct balanced composite curves: (STAVROS ANSWER)
Solution (STAVROS ANSWER)
• Define process stream system to be analyzed
• Extract stream data (Tstart, Ttarget, F*cp)
• Select a reasonable value for ΔTmin
• Perform cascade calculations. Determine QH,min, QC,min and TPinch. Draw GCC
• Select possible utilities, together with temperature levels and costs
• Using information about utilities and the process GCC, identify the cost-optimal utility
mix for the process (utility targeting)
• Include the utility mix in the process stream data and draw the resulting hot and cold
composite curves which are now balanced with respect to load
• The main use of the balanced composite curves is to estimate the heat exchanger area
required to achieve MER (area targeting)
True or false for GCC?
The hot and cold composite curves can be reconstructed
False. The GCC segments have merged (i.e., added and subtracted) the respective CC segments, which cannot be recalculated.
True or false for GCC?
The amount of potential hot utility savings can be read from the curve
False. The GCC curve allows to calculate the minimum hot utility requirements of the process, but it provides no information about the actual hot utility consumption.
True or false for GCC?
It is possible to perform utility targeting
True. This is actually one of the main advantages of the GCC, i.e., it allows not only to easily read the utility requirements (hot and cold) but also to identify the temperatures where these are needed.
True or false for GCC?
It is possible to perform area targeting
False. The merging of the CC curves to form the GCC does not allow to reconstruct them and draw the balanced composite curves, and thus the concept of vertical heat recovery cannot be applied to calculate the corresponding HEN area target.
True or false for GCC?
Given a set of available closed compression cycle heat pumps, it is possible to select the cycle that maximizes process hot utility savings.
True. The GCC can be used as the background process and the respective heat pumps as the foreground process. It by investigating all heat pumps, it is possible to select the heat pump that maximizes hot utility savings.
What happens with hot utility demand if ∆Tmin increases?
The hot utility demand increases when the global ∆Τmin increases.
3 Golden rules (pinch violation rules)
- Do not transfer heat through the pinch
- Do not cool with extrernal coolers above the pinch
- Du not heat with external heaters below the pinch
Eulers theorem:
U = N + L - S ( S = 1 for this version: Umin = N-1 ) U = number of units N = Number of streams, including hot and cold external utility levels S = Number of independent subsystems
For MER network: ( 2 sub-networks )
Umin,MER = Umin, above pinch + Umin, below pinch
What is an independent subsystem (S) ?
When one or more streams can satisfy demand for another stream in that part of the system.
Ex: (Below the pinch) Hot: 400kW + 600kW = 1000kW Cold: 1000kW Perfect match! (1 additional independent subsystem) S = 1 + 1 = 2
What is “area targeting”?
To estimate the heat exchanger area
required to achieve MER.
This is the main thing to use “balanced composite curves” (BCC) for.
What is “utility targeting”?
To identify the cost-optimal utility
mix for the process.
This can be done by using information about utilities and the process “grand composite curve” (GCC).
To find the utility requirments and the teperaturerequirments for the utilities.
Why do we want to place the heat pump around or above the pinch point?
What may be the problem with a bad placement?
The point of a heat pump is to reduce the demand for Hot Utility (Since it is usually much more expensive than cold utility). By placing this around or above the pinch point this is possible.
Placing it below the pinch point will not reduce demand for hot utility but might increse demand for cold utility with W,el.
Placing a heat pump in a heat pocket will most likely make the system less efficient.
