00:00:05.140 --> 00:00:11.450 Hi there, and welcome to this lecture on process flow diagrams: equipment symbol representations. 00:00:11.450 --> 00:00:14.130 My name is Marina Miletic. 00:00:14.130 --> 00:00:18.170 You may have seen these symbols before. 00:00:18.170 --> 00:00:22.039 These are heat exchangers, sometimes also called exchangers. 00:00:22.039 --> 00:00:27.659 If they're used to cool down a process stream, you'll also hear people refer to them as coolers 00:00:27.659 --> 00:00:30.039 or chillers as well. 00:00:30.039 --> 00:00:35.630 Heat exchangers have the letter abbreviation E. Unlike furnaces, heat exchangers rely on 00:00:35.630 --> 00:00:40.870 conductive or convective heat transfer across a surface, and not combustion of flammable 00:00:40.870 --> 00:00:42.800 gases. 00:00:42.800 --> 00:00:45.649 Let's look at these two symbols more closely. 00:00:45.649 --> 00:00:49.250 This symbol is a general, shorthand representation for a heat exchanger. 00:00:49.250 --> 00:00:53.340 There are two streams which go through a heat exchanger. 00:00:53.340 --> 00:00:57.870 One is represented by the black lines, and the other with the gray. 00:00:57.870 --> 00:01:03.469 This black line zig-zagging through the circle represents the utility stream. 00:01:03.469 --> 00:01:08.620 It's the job of the utility stream to heat up or cool down the other stream. 00:01:08.620 --> 00:01:13.840 The gray lines represent the process stream, which is whatever you're making or in the 00:01:13.840 --> 00:01:15.689 process of making. 00:01:15.689 --> 00:01:20.590 This stream is important because it's something you'll probably be reacting, separating, and 00:01:20.590 --> 00:01:22.920 eventually selling. 00:01:22.920 --> 00:01:28.299 This configuration represents a liquid or a gas utility stream going through a tube 00:01:28.299 --> 00:01:34.450 or a series of tubes, which heats up a liquid or gas process stream. 00:01:34.450 --> 00:01:40.540 In this configuration, the liquid or gas phase utility stream is cooling down a liquid or 00:01:40.540 --> 00:01:43.670 a gas phase process stream. 00:01:43.670 --> 00:01:46.439 Let's next look at some typical utility streams. 00:01:46.439 --> 00:01:52.500 There are about 8 common utility streams you'll see in a process flow diagram, and they each 00:01:52.500 --> 00:01:55.640 have a unique lowercase abbreviation. 00:01:55.640 --> 00:02:02.219 They include cooling water, abbreviated CW, which is water that comes from a cooling tower, 00:02:02.219 --> 00:02:07.400 and should be around 30 degrees centigrade when it enters the heat exchanger. 00:02:07.400 --> 00:02:13.730 Refrigerated water, abbreviated RW, is water that's at a temperature of around 5 degrees 00:02:13.730 --> 00:02:16.700 Centigrade going into the heat exchanger. 00:02:16.700 --> 00:02:22.560 Refrigerated brine, abbreviated RB, which is a dissolved salt in water at a temperature 00:02:22.560 --> 00:02:30.150 of around -45 degrees Centigrade going into the heat exchanger, low-pressure steam, abbreviated 00:02:30.150 --> 00:02:38.760 LPS, which is saturated steam around 140-160 degrees Centigrade going into the heat exchanger, 00:02:38.760 --> 00:02:46.310 medium pressure steam, abbreviated MPS, which is saturated steam around 180-200 degrees 00:02:46.310 --> 00:02:51.680 Centigrade going into the heat exchanger, high pressure steam, abbreviated HPS, which 00:02:51.680 --> 00:02:59.260 is saturated steam around 250-270 degrees 00:02:59.260 --> 00:03:06.840 electric heat, abbreviated EL, which uses either 220, 440, or 660 volt electricity to 00:03:06.840 --> 00:03:14.069 resistively heat a stream, and lastly, heat transfer medium, abbreviated HTM, which usually 00:03:14.069 --> 00:03:20.670 involves using a high-boiling point hydrocarbon to heat up a stream to around 400 degrees 00:03:20.670 --> 00:03:21.670 Centigrade. 00:03:21.670 --> 00:03:26.769 There are also a few other utilities you might see in a process flow diagram which don't 00:03:26.769 --> 00:03:32.989 have a specific letter abbreviation. 00:03:32.989 --> 00:03:37.129 Here are some examples of how these letter abbreviations should be used as labels for 00:03:37.129 --> 00:03:39.159 utility streams. 00:03:39.159 --> 00:03:44.819 Notice that the outgoing stream is not labeled since utility streams are only used for heating 00:03:44.819 --> 00:03:46.439 or cooling. 00:03:46.439 --> 00:03:51.909 When specifying them on a process flow diagram, something to keep in mind about all utility 00:03:51.909 --> 00:03:57.080 streams is that their temperature change across the heat exchanger should be greater than 00:03:57.080 --> 00:04:03.870 around 5 degrees Centigrade, but not more than about 15-20 degrees Centigrade. 00:04:03.870 --> 00:04:10.220 The reason for this is that if these streams are heated up or cooled down too much, this 00:04:10.220 --> 00:04:15.049 puts too much of an energy burden on equipment supplying the utility stream. 00:04:15.049 --> 00:04:19.970 In other words, the energy-intensive equipment involved in maintaining this utility stream 00:04:19.970 --> 00:04:23.070 would not be designed or used optimally. 00:04:23.070 --> 00:04:28.800 Also, a difference in temperature less than 5 degrees for a utility stream would require 00:04:28.800 --> 00:04:32.250 an excessively high utility flow rate. 00:04:32.250 --> 00:04:36.820 This would mean that we would need to purchase a larger than necessary steam boiler or cooling 00:04:36.820 --> 00:04:42.350 tower, because the temperature is only increasing or decreasing by a couple of degrees. 00:04:42.350 --> 00:04:48.250 So, in order to achieve the desired utility temperature change, increase or decrease the 00:04:48.250 --> 00:04:50.220 flowrate of that utility stream. 00:04:50.220 --> 00:04:55.470 Do this until you see that you're using just enough of that utility stream to create the 00:04:55.470 --> 00:04:58.350 desired temperature change. 00:04:58.350 --> 00:05:03.040 Keep in mind that the purpose of some of these utilities are to undergo a phase change, and 00:05:03.040 --> 00:05:07.460 either absorb or release alot of heat. 00:05:07.460 --> 00:05:13.310 This heat exchanger is a good representation of one you would see out in a plant environment. 00:05:13.310 --> 00:05:17.910 Shell and tube heat exchangers are important because they're the most common type of heat 00:05:17.910 --> 00:05:22.780 exchanger used in large liquid or gas process plants. 00:05:22.780 --> 00:05:27.300 If you've ever taken a tour of a chemical plant, you've probably encountered at least 00:05:27.300 --> 00:05:29.970 several of these. 00:05:29.970 --> 00:05:35.630 Shell and tube heat exchangers have two separate routes, called sides, for the two different 00:05:35.630 --> 00:05:37.070 streams. 00:05:37.070 --> 00:05:43.190 This route is called tube side, because fluid flows through the inside of tubes and transfers 00:05:43.190 --> 00:05:48.810 heat with the other route, called the shell side, where the fluid enters and surrounds 00:05:48.810 --> 00:05:53.810 the outside of the tubes to either cool or heat them. 00:05:53.810 --> 00:06:00.570 In this configuration, a hot fluid flows through the tube side, and transfers heat to a cold 00:06:00.570 --> 00:06:03.970 stream moving through the shell side. 00:06:03.970 --> 00:06:08.470 You might wonder, if you have two separate streams, how do you decide which one is sent 00:06:08.470 --> 00:06:12.520 through the tubes and which one is sent through the shell? 00:06:12.520 --> 00:06:17.690 A common myth is that the hot fluid should always go through the tube side, and the cold 00:06:17.690 --> 00:06:22.380 fluid through the shell, and this is wrong. 00:06:22.380 --> 00:06:28.750 Because the tubes can accommodate extreme pressure conditions better, corrosive, fouling, 00:06:28.750 --> 00:06:35.590 high or vacuum pressure fluids are usually sent through the tubes, so high pressure steam 00:06:35.590 --> 00:06:39.660 might be a good example of a fluid that's sent through the tubes, while the process 00:06:39.660 --> 00:06:43.330 fluid would be sent through the shell side. 00:06:43.330 --> 00:06:49.180 Fluids that need to condense from vapor to liquid, and viscous fluids are good candidates 00:06:49.180 --> 00:06:51.340 for the shell side. 00:06:51.340 --> 00:06:56.560 A good example of this would be sending the vapor from the top of a distillation column 00:06:56.560 --> 00:06:59.990 through the shell side of a condenser. 00:06:59.990 --> 00:07:04.470 Cooling water could go through the tubes, and the vapor process stream would condense 00:07:04.470 --> 00:07:07.530 on the surface of those tubes. 00:07:07.530 --> 00:07:11.370 Let's look at some other heat exchanger representations. 00:07:11.370 --> 00:07:15.840 These three are what you might see in a CHEMCAD simulation. 00:07:15.840 --> 00:07:21.190 The first two are what I would consider general, quick-and-dirty heat exchangers, and the third 00:07:21.190 --> 00:07:24.570 is a more realistic two stream heat exchanger. 00:07:24.570 --> 00:07:31.101 The grey lines for these heat exchangers are the process fluid, and the red lines are the 00:07:31.101 --> 00:07:33.720 utility stream. 00:07:33.720 --> 00:07:38.970 Notice that just because the red lines are drawn to look like tubes inside a heat exchanger, 00:07:38.970 --> 00:07:44.060 be sure to remember that utility streams are not necessarily tube side. 00:07:44.060 --> 00:07:50.750 You should decide which fluid is tube or shell side based on the chemical property guidelines. 00:07:50.750 --> 00:07:54.250 Let's look at these first two heat exchangers more closely. 00:07:54.250 --> 00:07:59.780 This upward pointing utility stream arrow means that this heat exchanger has a utility 00:07:59.780 --> 00:08:04.380 Stream whose job is to cool down the process stream. 00:08:04.380 --> 00:08:10.310 The cooling water comes out at a higher temperature than it came in, so this is why the arrow 00:08:10.310 --> 00:08:12.560 is pointing up. 00:08:12.560 --> 00:08:17.230 This heat exchanger has a downward pointing arrow, which means it has a utility stream 00:08:17.230 --> 00:08:20.800 whose job is to heat up a process stream. 00:08:20.800 --> 00:08:26.250 A good example of this would be steam: it's sent through the heat exchanger to heat up 00:08:26.250 --> 00:08:33.010 a process stream, and it comes out at a lower temperature. 00:08:33.010 --> 00:08:38.050 Like CHEMCAD, Aspen also has these two types of heat exchangers. 00:08:38.050 --> 00:08:44.510 One is a more general representation, and the other is a more rigorous two-stream heat 00:08:44.510 --> 00:08:46.010 00:08:46.010 --> 00:08:50.540 You might wonder how you can set up these heat exchangers so that they have a utility 00:08:50.540 --> 00:08:54.200 stream such as cooling water or steam going through them. 00:08:54.200 --> 00:08:56.580 Remember that these are simple heat exchangers. 00:08:56.580 --> 00:09:02.170 They are meant to be very fast and easy to set up in a process flow sheet. 00:09:02.170 --> 00:09:06.650 For this reason, you don't connect any utility streams to them. 00:09:06.650 --> 00:09:11.590 For this type of heat exchanger, there's no way to specify which utility stream you'd 00:09:11.590 --> 00:09:15.910 like to use, its temperature or flow rate. 00:09:15.910 --> 00:09:22.010 The purpose of this type of heat exchanger in CHEMCAD or Aspen is really just as a placeholder. 00:09:22.010 --> 00:09:25.830 I suggest you use these generic heat exchangers carefully. 00:09:25.830 --> 00:09:31.580 Believe it or not, both CHEMCAD and Aspen will allow you to heat up a stream to 10,000 00:09:31.580 --> 00:09:38.400 degrees, or cool it down to near-absolute zero using a generic heat exchanger like this. 00:09:38.400 --> 00:09:44.130 It's your job to go back and eventually replace these heat exchangers with ones that are better 00:09:44.130 --> 00:09:48.500 representations, such as the two-stream heat exchangers you see here. 00:09:48.500 --> 00:09:54.720 You can see with the more rigorous heat exchangers, you actually have to specify two streams coming 00:09:54.720 --> 00:10:00.940 in and going out, and depending on the properties of these streams, you may not be able to achieve 00:10:00.940 --> 00:10:03.590 your desired process temperature. 00:10:03.590 --> 00:10:09.080 You'll need to vary the flow rate of the utility stream, or use a utility stream with a different 00:10:09.080 --> 00:10:13.570 starting temperature to optimize your heat 00:10:13.570 --> 00:10:17.990 Remember that heat exchangers are there to achieve the optimum process temperature for 00:10:17.990 --> 00:10:23.030 other, more important pieces of equipment, and they're there to help keep the stream 00:10:23.030 --> 00:10:25.310 at a safe temperature. 00:10:25.310 --> 00:10:31.310 Design your reactors and separators first, because they're the foundation of your process, 00:10:31.310 --> 00:10:36.470 then go back and replace the generic heat exchangers with more realistic versions.
Office location
Engineering company LOTUS®
Russia, Ekaterinburg, Lunacharskogo street, 240/12