Building a boiler (6) – condensate return system

I’m using the type of return system used in almost every dry cleaning/laundry operation called an “open loop return system.”  There are other types of return systems, but they are seldom, if ever, used for our applications.

Like the steam delivery system, the condensate return system also has drops between the individual machines and a large main pipe running back to the boiler return tank called the “return header.” Connected to the outlet of the steam trap is a “check valve” of some sort, usually of the “swing check” family. The check valve is used to prevent the flow of condensate back to the outlet side of the steam trap.

There should also be a ball-type shut-off valve after the check valve, used to isolate the return system from the check valve and trap. Unfortunately, these shut-off valves are not present in all installations. If these valves are not currently installed, why not install one when you change out a steam trap? They are cheap, open quickly, and are easy to install, providing a safe and dry environment when changing traps.

The condensate is pushed back into the “condensate return tank” through the return header. The condensate return tank is a major part of the steam system and performs so many functions that we’ll honor it with its own paragraph.

CONDENSATE RETURN TANK:

The “condensate return tank” is a large vessel vented to the outside (some are cylindrical, others rectangular in shape, and some are built around the boiler shell), all with the “boiler return pump” located beneath. The condensate return tank is a multipurpose device which:

1. Acts as a reservoir or catch basin for the condensate returned from all of the steam-heated machinery, providing a source to flood the intake side of the return pump, which pushes water into the boiler
2. It contains a device that supplies new water to make up for the condensate lost due to venting, evaporation and the use of live steam in the finishing and spotting of garments. In most cases, this is a ball-float valve operated like the float valve in most toilet tanks.
3. A convenient mixing and distribution system for the addition of chemicals to the boiler, such as boiler compound and/or other additives, which prevent harmful scale from building up, which clog the boiler tubes as well as scavenge (trap and bind oxygen which is naturally dissolved in make-up water) preventing the formation carbonic acid (H2CO3) the same stuff that tickles your nose when you drink soda water (seltzer) too fast. In carbonated drinks, it may only tickle your nose, in steam and return systems it dissolves steel and iron (as in pipes or tanks) over a relatively short period of time.

For now, I’ll just discuss item number 1 on the previous list, the reservoir aspect of the condensate return tank. A preset level is maintained in the return tank by adding make-up water, often called city water (water supplied by an outside source), to the condensate returned by the machinery into the return tank.

Water is pumped from the return tank to the boiler by sensing the level in the boiler and turning the return pump on and off. At the outlet of the return tank, there will likely be a “Y” type strainer. The strainer traps particles of scale, rust, corrosion, or the occasional nut or bolt breaking loose and arriving at the return tank. After the “Y” strainer, there is usually a “turbine” type return pump, sometimes called the “condensate feed pump.”

Turbine pumps are designed to push a low volume of water against a high head pressure. This pump is required to refill the boiler with water as the water is turned into steam, distributed throughout the plant. After the feed pump outlet, there are generally a series of check valves (valves that allow flow in only one direction). On many boilers, the first check valve is a stainless steel “spring loaded metal disk” type check valve designed to pass large pieces of debris without being jammed into an open position. The next check valve is usually a “swing check” or another spring-loaded check made from bronze with a Teflon disk. The purpose of all of these check valves is to prevent water from being pushed out of the boiler, through the pump, into the return tank when the pump is off. Return pumps and check valves are the most problematic components in your steam system.

Well, that’s it for this month. Next month, we will deal with troubleshooting common boiler problems. Look for a downloadable PDF file of this article on my FIXIT-ACADEMY website www.fixit-academy.net, where even an amateur can learn to make your own equipment repairs.