Satsha Management Services has designed for one of its esteemed clients the shell-and-tube heat exchanger that handles a fluid with viscosity as high as 17000 cP at 40 ºC. The exchanger cools molasses, a by-product of sugar manufacturing process, from 50 ºC to 40 ºC with cooling water available at 37 ºC. The Process conditions of hot fluid (Molasses) are as under:
Heat duty 83720 KCal/hr
Flow rate 15 m3/hr
Inlet temperature 50 °C
Outlet temperature 40 °C
Density 1400 kg/m³
Viscosity at 40ºC 17000 cP
Viscosity at 50ºC 6000 cP
Specific heat 2500 J/kg K
Thermal Conductivity 0.3 W/mK
Maximum pressure drop 2 bar
Design Constraints
The given Process conditions set following constraints for the design of the exchanger:
i) The extremely high viscosity of molasses (17000 cP at 40 deg C) makes the design of the exchanger pressure-drop-limited.
ii) As molasses side is controlling the heat transfer and its heat transfer coefficient is extremely low due to extremely high viscosity, the exchanger design is limited by heat transfer also. It is to be noted that such a high viscosity leaves little room to increase the velocity due to high pressure drop. Thus, it also causes very low velocity on molasses side. The low velocity reinforces the impact of high viscosity on the heat transfer coefficient. The combined impact of the two factors makes the molasses side heat transfer coefficient very low.
iii) As approach (temperature difference between molasses outlet and cooling water inlet) is very low (3 deg C), it makes the exchanger MTD (mean temperature difference) limited too. In fact, shell and tube heat exchangers are rarely designed for an approach less than 5-6 deg C. As below this limit, a shell and tube heat exchanger becomes highly inefficient from heat transfer point of view. Yet, as per the client’s requirement, Satsha Management Services designed the shell-and-tube exchanger with lowest possible heat transfer area.
Design Philosophy
As molasses side heat transfer coefficient is very low (due to excessively high viscosity) and it is controlling the heat transfer (the other side being cooling water with high heat transfer coefficient), the overall heat transfer coefficient becomes very low. This requires large heat transfer area for the exchanger even for a small heat duty of 83720 kCal/hr.
As molasses has extremely high viscosity, it has been placed on shell side. Putting on shell side and providing 45○ tube lay out (rotated square pitch) creates the tendency for turbulent flow leading to higher heat transfer coefficient and lower required heat transfer area as compared to placing it in tubes. Putting it on tube side would make the flow laminar resulting into very low heat transfer coefficient and making the exchanger excessively large.
In order to ensure a reasonably high velocity of cooling water in tubes, the number of tube passes has been increased to 8. The high velocity in tubes minimizes the fouling tendency of cooling water.
As molasses is supposed to have fouling tendency, U-tube exchanger has been selected to facilitate mechanical cleaning of outer surface of the tubes by way of removal of tube bundle. A minimum cleaning lane of 0.25 inch as required by TEMA has been ensured by specifying tube pitch 1 inch for ¾ inch tube outer diameter.
A 45○ tube lay out (rotated square pitch) has been selected which provides dual advantage in this case. Firstly, it creates induced turbulence on shell side and hence improves shell side and overall heat transfer coefficient particularly when viscosity of molasses is so high. Secondly, it facilitates mechanical cleaning of outer surface of tube bundle.
Double segmental type of baffle has been used to cope up with high pressure drop on molasses side due to its extremely high viscosity. With double segmental baffle, cross flow in shell is divided and hence shell side pressure drop is reduced substantially.