Heat Exchangers- PHE vs. SSHEX vs. Shell & Tube. Which should you use?
As most who have been following this blog know, at Triplex, we specialize in sanitary pumps. We do hundreds and hundreds of pumps each year. At the end of the day, a pump moves fluid from Point A to Point B and usually through a few pieces of equipment in between. In today’s post, we wanted to talk a little bit about one of the more common “something in between ’s”- heat exchangers. Specifically, the three most common types of heat exchangers we see in high purity process applications- Plate Heat Exchangers, Shell & Tube heat exchangers, and Scrape Surface heat exchangers. In this post, we will provide an overview of each technology and help you decide which one you should use for your high purity process application.
Plate Heat Exchangers
Let’s start with one of the more common technologies we see in almost every dairy in the world- plate heat exchangers. Plate heat exchangers include a series of corrugated parallel plates separated from each other by gaskets. Gasketed plate heat exchangers are so efficient because they use plates and gaskets to guide the flow of heating and cooling fluids in countercurrent flow with a very small gap, enabling high efficiency heat transfer over a small space.
APV’s industry leading plate heat exchangers are ideal for simple, clean fluids that don’t contain particulates. And as mentioned above, Plate Heat Exchangers or PHE’s do the best job of packing a large amount of heat transfer space into a small footprint. PHE installs are typically ½ the footprint of comparably sized scrape surface and shell and tube units. They also allow for multiple sections, allowing one unit to be able to handle heating, cooling, and regeneration. PHEs can be cleaned in placed, but can be challenging to service- requiring re-gasketing and replacement plates- which are not so much expensive as time consuming. They are also limited on temperature and pressure.
Shell & Tube Heat Exchangers
While Plate Heat Exchangers use parallel plates, shell and tube heat exchangers use a tube bundle that is submerged in the heating or tubing medium. We see shell and tube units most commonly in pharmaceutical applications or medium to high viscosity applications. Because tube diameters are larger than the gaps between plates in a PHE, we can handle larger particulates, but need much more space to do so. As with all heat exchangers, shell and tubes are designed for heat and cooling media to flow countercurrent to on another and newer designs have features like baffles for turbulent flow to enable maximum heat transfer.
As mentioned above, we see shell and tubes most commonly in pharmaceutical applications for CIP and WFI. There are a few reasons for this, including their simplicity, excellent drainability, high surface finish options for maximum cleanability, and double tube sheet options. Double tube sheet designs eliminate the chance of cross contamination by routing leaks to the joint in the outer tube plate. Shell and tubes are our mid-price option and carry lower maintenance costs than scraped surface designs. They handle high temperatures well and have excellent drainability. Disadvantages include inability to be expanded and the larger overall footprint and maintenance cost compared to similarly sized PHEs.
Scrape Surface Heat Exchangers
The last type of heat exchange technology we will take a look at are Scrape Surface Heat Exchangers. Specifically, Votator Scrape Surface Heat Exchangers. Scrape Surface Heat Exchangers, or SSHEX, are a little like a shell in tube in that they have a jacketed outer tube for heat transfer media and an inner tube through which product flows. The key difference is that SSHEX feature a central mutator shaft with pinned blades and is driven by a motor to scrape the walls of the tube. Not only does this prevent burn on and product fouling, it also enables even and controlled heating and cooling for exceptionally high viscosity products.
As we alluded to above, SSHEX are ideal for high viscosity products and products containing large particulates. The large central annual space helps minimize pressure loss through the units. They can be used in both heating and cooling applications and jackets are ASME rated and allow the use of both liquid (hot water or glycol) as well as gas (ammonia or steam) as heat transfer media. Disadvantages include larger floor size, initial cost, and ongoing maintenance costs. That being said, for high viscosity products, SSHEX are the only real solution we offer.
Which heat exchanger is right for you?
So, which SSHEX should you use? Now that we’ve given an overview of the three primary technologies, we offer, let’s recap when each should be used. For simple, every day, low viscosity, clean fluid applications, we will recommend a plate heat exchanger. Plate heat exchangers deliver best value of any of the three types of technologies we offer and also allow for future expansion. The PHE has been will and will probably always be a staple in breweries and dairies across the world.
For higher purity applications, particularly where we want to use steam as a heat transfer medium, we will recommend a shell and tube heat exchanger. A shell and tube heat exchanger allows for the lower surface finishes required in pharmaceutical applications and feature double tube sheet designs to prevent cross contamination. They can be pitched for complete drainability and are relatively simple to service vs. a PHE or SSHEX. Certain shell and tube designs can also handle slightly higher viscosities than a PHE. We’ll often see these used in hot and cold WFI (water for injection) applications.
SSHEX are for when we need to pull out all the stops on high viscosity products. While they’re significantly more expensive than a PHE or Shell and Tube, the Votator’s larger inner tube and scraping mutator shaft allow us to evenly heat and cool high viscosity products with minimal pressure drop through the system. We love applications where we can use a Votator to prechill a product prior to a cooling wheel or help transition from a batch to continuous processing of a product to create huge ROI opportunities for customers. We see SSHEX run on everything from shortening to tzatziki sauce and peanut butters, as well as pet food and even fish guts!
So when considering a new heat exchanger, think first about the characteristics are your product- is it thick or thin? Does it contain large particulates? Also consider how you will clean the unit and any industry regulations you may be subject to. Finally, think about where the unit is going to go and what you’ll do if you need more capacity moving forward. And as always, if you have any questions about which heat exchange technology is right for you, contact a Triplex Sales Engineer today!