What is a shell and tube heat exchanger?
Take a walk through virtually any industrial facility that requires the process fluid to be cooled or heated and there is a good chance you will come across shell and tube heat exchangers. Shell and tube heat exchangers are one of the most common types of heat exchanger due to the fact that they’re relatively inexpensive to manufacturer, require no external energy to operate and enjoy long term reliability, thanks to the absence of moving parts.
Shell and tube heat exchangers are compatible with both liquid and gas, giving them a huge range of suitable applications. It’s important to remember when discussing heat exchangers that a “fluid” can be in a gaseous or liquid state. Technically, it is not proper terminology to call a heat exchanger a cooler because the process fluid can be warmed as well as chilled. For our purposes, we will focus on the cooling capabilities of a heat exchanger. That is what RHM deals with in fluid power and lubrication applications.
Components and design of a shell and tube heat exchanger
While various design subtleties exist for specialization in application, shell and tube heat exchangers contain a few consistent features:
- Shell: The shell is what you would first notice when you come across a shell and tube cooler. This is the outer portion that holds the process fluid and houses the internals.
- Tubes / tube bundle: The tubes contain and flow the cooling medium. Tubes must have excellent thermal transfer properties, be resilient to large temperature fluctuations / differentials and resistant to corrosion. The summation of the tubes referred to as a tube bundle or simply stated: the bundle. In many designs, the bundle may be removable for maintenance and service purposes.
- Tube material is an importance consideration when selecting the heat exchanger. The tubes see a lot of stress due to the fact they are subject to a large temperature differential. Common tube materials include copper, brass and various steel alloys.
- Four ports: Connections that provide hook ups for the process mediums. These ports are typically raised face flanges or NPT type connections.
- Shell side fluid in
- Shell side fluid out
- Tube side fluid in
- Tube side fluid out
- Inlet / Outlet Plenums: Found on the end(s) of the shell. This is the open area where the tube bundle either collects or discharges the cooling fluid.
- Baffles: The baffle creates turbulence within the shell to improve efficiency and reduce “concentrations” of hot or cold. Baffles are simple design considerations but essential to proper function.
- Pressure Differential: It is common for there to be a pressure differential, so in the case of an unexpected leak, the cross contamination damage is minimized. Typically you would see the pressure being greater in the tube sheet / shell, so if there were to be an unexpected leak, the process fluid would flow into the cooling medium. This design prevents the process fluid from being contaminated leading to costly accidents and/or failures.
So how does a shell and tube heat exchanger work?
Put rather simply, there are two fluids of different temperatures in the cooling operation, one being the process and the other being the cooling medium. The process fluid that is being cooled generally runs through relatively small diameter tubes that are housed within the shell. The outer shell contains and circulates the cooling medium. Both process and cooling fluid are continuously kept in circulation for the heat exchanger to function properly. In order to be efficient, shell and tube heat exchangers utilize round small tubes creating a large surface that doesn’t take up unnecessary room.
Cooling Fluid Options:
Considerations must be made when choosing a medium to cool with. In most situations the cooling medium that is chosen is the one that’s available. Most plants that require the use of shell and tube heat exchangers have a water supply or some form of network that pumps a synthetic coolant mix throughout the premise. A few of the most common mediums:
- Water: Effective in most applications, water is the most common fluid used to cool. Plain water is readily available, low in cost to use and exhibits sufficient, if not great, thermal transfer qualities.
- Ethylene glycol: EG is a synthetic compound used in various applications where the cooling demands may become more extreme. EG is typically mixed with water to create a coolant mixture that has a higher boiling point and lower freezing point. EG is what you would find in your car’s coolant system and it is very toxic.
- Propylene glycol: The big differentiator between EG and PG is the fact that PG is markedly less toxic. PG is commonly used in applications where the toxicity of the coolant becomes an issue. The thermal transfer, boiling and freezing point modification qualities are very similar if not the same as EG.
- TEMA: Stands for Tubular Exchanger Manufacturers Association. TEMA is a set of standards that helps define the manufacturing tolerances and machining used when constructing a shell and tube heat exchanger. TEMA helps the end user know that his heat exchanger meets industry standards and is built in a fashion that reflects quality. There are three common classes of ratings:
- TEMA C – General Service
- TEMA B – Chemical Service
- TEMA R – Refinery Service
When evaluated, TEMA C is the least strict and TEMA R is the most stringent set of guidelines.
- ASME: Stands for American Society of Mechanical Engineers. ASME is a broad set of codes and standards that is focused on mechanical engineering, as the name implies. When discussing shell and tube heat exchangers we are interested in the ASME standard Boiler and Pressure Vessel Code (BPVC). Similar to TEMA, ASME BPVC serves as a benchmark for the construction and quality of among many other pressure containing vessels, heat exchangers. A heat exchanger that bears an ASME code stamp will be more costly but is guaranteed to be a quality unit.
Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.
Experts in fluid power, electrical and mechanical technologies.