In an air removal system, the condensate from the inter/after condenser is normally returned to the main condenser’s hotwell. This is done for a few reasons. The main condenser hotwell runs at a deeper vacuum so a pump is not required to remove the condensate when the unit is running. The condensate is made up entirely of clean boiler steam, so returning it to the main condenser hotwell allows it to be returned as feed to the boiler which saves both on energy costs and boiler feed water treatment.

Two things can happen with the condensate drainage that would result in a performance problem. The condensate can struggle to drain, flooding the corresponding inter or after condenser. When this happens, the ejectors have to push the air load through a liquid level and they will struggle to perform. This will have a negative impact on the turbine backpressure. The other issue occurs when both air and condensate is allowed to be pulled back to the main condenser when one of the condensate lines is not sealing properly. When this happens, the air that is being evacuated ends up being recycled back to the main condenser, which adds an additional load to the air removal system. The recycled load results in a poor turbine back pressure as the venting system will behave as though it is overloaded.

There are different ways to set up the condensate drain lines for the inter and after condensers. They can sometimes differ because the intercondenser operates at vacuum where the aftercondenser does not. In addition, it’s important to note that each condenser should have independent condensate drain lines. Tying the intercondenser and aftercondenser drain lines together often causes performance issues.

Loop Seal – A loop seal can be used for the intercondenser given the minimal pressure differential between the main condenser and inter condenser. Normally a loop of 10-15 ft is adequate, although the require loop height should be calculated. As long as the loop seal is primed and the system operated within its design, loop seals are very reliable.  After many years of use, debris can gather at the bottom of the loop and may require the loop be cleaned out.

Trap – A float trap can be used for this service. The trap will allow only condensate to drain back to the main condenser. While this does introduce a mechanical element that is more prone to failure than a loop seal, it does allows for less piping or installation where a 10-15 ft loop seal is prohibitive.

Trap – A float trap can be used here in the same manner it would be used in the intercondenser. Given the larger pressure differential between the aftercondenser and main condenser, a 35 ft loop seal is usually prohibitive and returning condensate requires a trap.

Drain – Because the aftercondenser runs at a near atmospheric pressure, the condensate from the aftercondenser can be directed to a drain. While this is very reliable in preventing flooding and eliminates the possibility of recycle air, the condensate is not recycled when it is put to a drain.

Condensate traps have some installation caveats that needs to be understood. They should be installed at least 18 inches below the inter/after condensers, as they can require a small amount of liquid head to function correctly, especially at start up. If they are installed right at the bottom of a condenser, it can cause condensate to back up. Drain traps also should be installed with a bypass line and proper isolation valves so that the trap can be isolated and worked on when it needs servicing. Many of the ball and float traps used for this service come with a thermostatic valve. These thermostatic valves are not needed for this service, any thermostatic valves should be removed and the connection should be plugged. Failure to do this can cause air to leak through the thermostatic valve and recycle, causing a reduction in performance. In an air removal service, the float trap does not require a balance line. The differential pressure across the trap will drive the condensate towards the main condenser and the condensate piping is purposely oversized to allow the lines to self-vent, making vapor lock unlikely. Therefore, the traps do not require balance lines for an air removal service.

When piping up the condensate lines for the inter and after condensers, simple is better. An effort should be made to ensure that the piping continually slopes downward towards the main condenser hotwell. The piping should avoid as many 90 deg elbows as possible. It should also be stated that the inter/after condenser should be installed above the hotwell level of the main condenser. If the air venting system is installed below main condenser, an entirely different condensate arrangement will be required.