This is maybe a catchy title but this is what a title should be about, isn’t it. This post is the first post about the importance of rigorous calculation compare to the simpler approach from a simulator. Don’t get me wrong simulators are very good and many processes engineers are using them every day but they do have limitation and it is the job of the engineers to know what these limitations are and how to overcome them. I created a very simple case of a heat exchanger in one of the most famous simulator in the oil and gas industry HYSYS.
People familiar with the tool will recognised that the tool has calculated the heat exchanger and that everything is fine. From the simulator point of view and an ideal mass-heat balance this is correct. We have a stream coming in at 90C and leaving at 20C, on the other side we have the exact opposite. So in summary all the heat from side is transfer to the other side, and all this without any pressure drop. Now if you give this specification to a heat exchanger expert, he will struggle to design such an exchanger. Lets have a look at why.
The first aspect which I see quite often forgotten is pressure drop. When a fluid is flowing through equipment, it has to lose pressure. In the simulator you can specify the pressure drop that you want and 0 is perfectly adequate. This means that either you expect it to be very small and that you plan to add some equipment to compensate for the loss of pressure somewhere on the line. However when you start looking at a more detailed view of the equipments the pressure drop has to be taken into account. In a very simplistic view, you can say that the pressure drop is used to create heat transfer. So the more pressure drop you have, the more heat transfer you have.
The second aspect is that the world is far from ideal... I won’t go into the details of how the heat transfer works but not all the energy from one stream can go to the next. It has to cross materials, loose heat to the outside of the equipment...
But more importantly, you need a driving force. The smaller the temperature difference is the smaller the quantity of heat transfer will be, in other terms, the smaller the temperature difference is, the bigger the exchange surface needs to be to exchange the same quantity of heat. It is a bit like if you try to reach 2 when you start at 1 and add half of the previous number 1 + 0.5 + 0.25.... Mathematically you will reach 2 only after an infinite number of additions. So in theory you will need an exchanger with an infinite surface. Each different type of exchanger needs different driving force. The engineer needs to choose the correct exchanger for the correct job. In a “normal” shell & tube exchanger it could be 5C when in a specialized plate fin exchanger it can go down to 0.1C
So a more realistic exchanger will be
This exchanger has some pressure drop on both sides, and a minimum temperature approach of 5C.
