Continuous Process and Bath Process
All productive processes can be classified according to how the raw material input stage is carried out, and how the product is subsequently obtained. These processes are basically divided into continuous processes and batch processes, although we can find variations combining features from both processes.
- Continuous process. The incoming flow of raw material to the system is constant during the production cycle, being the product output constant as well.
- Batch process. The total amount of raw material is introduced to the system at the beginning of the process, in such a way that the total outcoming product is obtained after a certain time.
In those processes requiring a heat input or output in the product, the energy consumption and the duration of the process is different whether it is carried out continuously or if it is done in batches.
In addition, the type of process determines the size of the required heat exchanger.
The following provides some of the common reasons why a batch process is selected instead of continuous process:
- The product to be processed, or the hot/cold service fluids, haven’t got a steady availability.
- A certain residence time is required in the process, for example, in order to allow the stabilization of the properties of the product or to allow for developing chemical reactions.
- The maintenance and / or cleaning tasks bring about a significant time.
Example. Let us suppose that a consumption of 10.000 kg/h of hot water at 85 ºC is required for a certain production process, whose duration is 30 minutes. For this purpose, mains water is available at 20ºC and steam is available at 4.5 barg (155ºC) as a heating medium. The heating is intended to be performed by means of a tubular heat exchanger.
The flow of hot water is fed constantly throughout the process.
Figure 1. Diagram of a continuous process with a heat exchanger.
The total water consumption will be 10.000 kg / h x 0.5 h = 5.000 kg.
The required steam flow at 4.5 barg, according to the energy balance, is 1.300 kg / h, which remains constant throughout the entire process.
- Therefore, the total steam consumption is 1.300 kg / h x 0.5 h = 650 kg.
The tubular heat exchanger should be able to heat the water flow from 20 ° C up to 85 ° C, in an instantaneous process.
- Firstly, the designer must select a shell diameter with a number of inner tubes suitable for the water flow.
- The length of the tubes shall ensure a sufficient exchange area, according to the equation Q = U x A x LMTD, in which Q is the exchange power, U is the overall heat transfer coefficient, A is the exchange area and LMTD is the average logarithmic temperature
- For important temperature raises, as in the current example (where the heating is performed from 20 ºC to 85 ºC), it is usual for the heat exchanger to require a large nominal length, as for instance, 3.000 mm.
The hot water mass is supplied before the process begins.
Figure 2. Diagram of a batch process with a heat exchanger.
To carry out the batch process, a vessel will be required, having the capacity to store the total mass of water of 5.000 kg. Therefore, the total water consumption is the same as in continuous process. But there are differences with respect to the continuous process, as shown below:
- In a batch process, it takes a certain time t1 to store first the total mass of water at 20 ºC in the tank
- A certain time t2 is then required to heat that mass of water, from 20 ° C to 85 ° C. It will depend on the recirculation flow, which will not necessarily be the same than the consumption flow rate (10.000 kg / h in this example).
- The overall operation time for the batch process will result of adding the duration of the process (30 minutes in this example) plus these previous times: that is, t1 + t2 + 0.5 hours. However, in the continuous process, the total operating time matches with the duration of the process, 0.5 hours.
If we do the energy balance, the required steam flow at 4.5 barg is greater at the beginning of the recirculation, when the water is cooler (at 20 ° C at the start), and it is lower and lower at each step or increment as the water in the tank warms up.
- Given that a maximum heating time t2 of 1 hour is established and a recirculation flow rate of 7.500 kg / h is selected: in this example, to heat 5.000 kg of water from 20 ºC to 85 ºC, 700 kg / h of steam at the beginning of the recirculation, and 410 kg / h at the end.
- Although the decrease in steam flow over the process is not a linear reduction, the total consumption can be approximated as the average value of the initial and final consumption: ½ x (700 kg / h + 410 kg / h) x 1 h = 555 kg. In contrast with a continuous process, the reduction of steam consumption is important, 14.6%: 555 kg instead of 650 kg.
Unlike the continuous process, the tubular heat exchanger in batch is not required to raise the entire temperature difference from 20 ºC to 85 ºC instantaneously, but it will perform partial temperature rises instead, getting these increments smaller and smaller, until completing the heating process.
- In general, if we keep the same shell diameter and the same number of inner tubes as in the continuous process, the nominal length of the heat exchanger for the batch process will be notably lower.
- It is common for the batch process to require a heat exchanger of 1.500 mm of nominal length (half of that for the continuous process in this example), resulting in a lower cost.
SACOME has a range of standard shell and tube heat exchangers of 1.500 mm of nominal length, our V SERIES, for water heating and cooling applications.
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