Shown above is a cross sectional sketch of a typical Graham built vacuum condenser. The internal configuration is arranged and optimized for condensing. This is accomplished by incorporating vapor space above the tube bundle. Tube support plates are incorporated not to direct flow, but only to support the tubes. Internal geometry of the unit is designed to minimize pressure drop of the shell side vapor. This helps ensure an efficient condensing is achieved.
“Why is a heat exchanger design not suitable in a vacuum system?” This question is sometimes asked if the fundamental difference between heat exchangers and vacuum condensers is not understood. In order for a vapor to condense, that vapor only needs to contact the vacuum condenser’s cold tubes once. At that point it changes form and becomes a liquid, dropping out of the process flow. Transferring additional heat into the condensed liquid has no additional benefit. The vacuum condenser internal arrangement is built around this idea. A heat exchanger’s internals are designed to funnel the process across multiple tube surfaces to transfer heat, not to facilitate condensation. Due to the internal differences the shell side pressure drop across a heat exchanger is much higher than a vacuum condenser.
In a vacuum system pressure drop is problematic. The additional pressure drop means the vacuum system’s ejectors need to be designed for a higher compression to overcome the heat exchanger’s excessive pressure drop. This results in the system needing larger ejectors that use significantly more steam. In an existing system, replacing a vacuum condenser with a heat exchanger often results in the upstream ejector not being able to overcome the additional pressure drop, resulting in poor performance. In a system with a long turnaround cycle this can become extremely troublesome and expensive to correct, which is why it is extremely important to understand that the two differ.
Vacuum condensers cannot be easily modeled with commercially available heat exchanger software. Existing programs do not do a good job of modeling vacuum condensers because they are tailored to heat exchanger design, not vacuum condenser design. Heat exchanger guarantees are normally in terms of heat transfer, where vacuum condenser guarantees are normally in terms of load and operating vacuum.
Hopefully you will look at vacuum condensers and heat exchangers differently after reading this. Remember, if you have any questions about your system or your vacuum condensers please don’t hesitate to contact Graham. We are always glad to answer your questions and provide assistance.