Hi there, and welcome to this lecture on process flow diagrams: utility stream tables.
My name is Marina Miletic.
There's one aspect of stream tables that's important to discuss, and it has to do with
utility streams.
You may have noticed that on this process flow diagram, there are four heat exchangers
using utility streams.
Two of these use high pressure steam, one uses cooling water, and another uses freon
refrigerant.
But what are the temperatures, pressures, and flowrates of these streams coming in and
going out of these heat exchangers?
On this diagram, there are no stream numbers for these utility streams, so there would
be no place to provide this information.
Because this diagram is meant as a very simplified representation of this process, I did not
specify or number the utility streams.
A better, more detailed representation of the same process would be this process flow
diagram.
If we now look more closely at these heat exchangers, all streams, including the ones
used for utilities, are numbered.
The process streams are numbered 1 through 21, and the utility streams are numbered 22
through 29.
Notice that the utility streams are the last ones numbered because the process streams
take priority.
The main reason for this is to organize all utility streams sequentially and in one block,
so that they're easier to read from the table.
If these streams were numbered such that they were alternating between process and utility
streams, it would be much harder to track the temperature changes in each one when reading
information from a table.
This is why it makes sense to number process streams first, and in the order of the path
of the process, and utility streams last.
In terms of presenting the physical properties of each of these utility streams, there are
two choices.
You can either incorporate the utility streams in the same table as the process streams,
and include the same level of detail as you would see in a stream table, or better yet,
you would put these utility streams in a separate utility stream table, also sometimes called
a utility summary table.
This is a better option for a few reasons.
Let's look at an example where the utility streams are incorporated into a stream table
to see why.
This is the stream table for our process.
Notice these are the process streams, and these are the utility streams.
Notice that utility streams usually don't have a complicated composition with many individual
flowrates in each stream.
They are usually made up of only one component, such as water or refrigerant, so it's not
necessary to provide details on the individual concentrations of many different chemicals
in each stream.
Related to this, if you incorporate utility streams as part of the stream table, it is
hard to tell just from looking at this table if the water listed here is steam, cooling
water, refrigerated water, a reactant, or a product in the process.
It could be involved directly in making the product in the process or not.
You would need to flip back to the report to confirm this for sure.
There is one more issue that might make this table confusing.
Other information you might want to provide on a stream table, such as density, heat capacity,
or viscosity are not as important for utility streams, so this adds some unnecessary information
and bulk to the table.
For all of these reasons, a separate utility stream table is almost always a better choice.
Notice there are fewer rows in this table and less unnecessary information.
These tables tend to be much neater and more concise than tables which feature process
and utility streams together.
For this reason, I recommend presenting your utility streams using a table like this.
Let's next talk about some things to keep in mind when setting up a utility stream table.
Be sure to label and number your utility stream table as a separate table.
Provide a brief description and reference the process flow diagram where the stream
numbers are shown.
Notice this table is labeled "Table 2" because Table 1 is the stream table.
Utility stream tables only present the utilities in the process, and all the other components,
such as reactants, intermediates, and products are not listed.
This helps make the table much faster and easier to read.
Also notice that steam, cooling water, and refrigerated water are all listed as water.
Any kind of steam, such as the high pressure steam used in streams 22 through 25 is listed
as water, and cooling water, which is used in streams 26 and 27 is listed as water as
well.
If we had refrigerated water in this process, we would also list it as water.
Do not list these as steam, cooling water, or refrigerated water in the table.
if you're wondering then, how would somebody be able to tell if you're using high, medium,
or low pressure steam, or cooling water, or refrigerated water as the utility?
This is determined by looking at the temperature, pressure, and vapor fraction of the water.
We can deduce the type of utility being used through these physical properties.
As with any stream table, be sure to use appropriate units so that you avoid numbers in the tens
of thousands.
Avoid presenting lots of significant figures or dominating the table with scientific notation.
Be sure to use commas when necessary, and align numbers so that they are as easy to
read as possible.