Pressure Vessels, Pressurization Units and Decoupler Headers

This article may be helpful. It explains about decoupling header in the context of HVAC chiller sytems.

http://www.districtenergy.org/pdfs/DEMagArticles/4Q06/4Q06InsideInsights.pdf

Thank you for the informative link, I read it and learned some things that I was never in a position to be involved with since I hadn't been working on systems this large. Referencing the problem that the author alluded to near the end of the article, how about if a special fiting was made to distribte the bypassed water somewhat evenly such as a modified venturi design based on a carburator. This could have the potential to greatly reduce the length of pipe needed for complete mixing, enabling more dsign flexibility where space is tight. Please let me know if this sounds feasable.

When I read the article, I wondered along similar lines. I was thinking about something like mixing vanes in the pipe, or a static mixer installed inside. I am not familiar enough with these ideas to assess what might really work, or what would exist in such large sizes. That's one helluva chiller header at 42".

Hi Tornado,

The web link is interesting but it stops short of giving the real world 'cast iron' solution. The weakness as the author identifies is in the relative flows in the chiller circuit and the load circuit. I have experienced this situation with multiple chillers and widely varying load flow rates. One of the key issues with chillers unlike heating plant is the small delta T available and therefore the loss in performance which occurs when 'un-cooled' system water starts to infiltrate the chilled water leaving the chillers. A small diameter byepass pipe as shown may be used to provide information about the relative temperature existing in it and the chiller flow and system return. Ideally you want a small reverse flow with a 'blend' taking place at the junction of the system return and the connection to the chillers. I would always define this as being the 'Kirchoff' point; and as such the point at which the pressurisation/expansion vessel connection would be made.

The big question is how best to control the plant to guarantee that the chiller exit temperature water is always presented to the system pumps without dilution. An intelligent bit of control logic helps, the transition from one chiller to another is difficult to achieve without load shedding and possible instability in the control loop.

Having experimented with various set-ups over the years I favour introducing a thermal buffer in the 'balance pipe', and of making it have a height to diameter ratio of 3:1. Care must be taken with the return connection orientation to prevent creating too much disturbance. Unfortunately the volume must be large, we are into the tonnes of water to be of any real benefit in the argument.

Good chat,

Massey.