Condensate pump

01

Avoid cavitation with hot condensate

Spirax Sarco, a steam systems maker, explains that condensate forms close to its boiling point. A centrifugal pump has a low-pressure zone at the eye of its impeller, so near-boiling condensate can flash into steam bubbles there; as the bubbles move outward into higher pressure they implode, which is cavitation, a noisy process that damages pump internals. It therefore recommends pumping condensate with electric pumps built for the task, and keeping condensate in atmospheric systems at or below 98 °C. Some pumps are limited to 94 or 96 °C, depending on design, speed and the height of the receiver above the pump. [1]

Spirax gives the vapour pressure points behind this: water boils at 100 °C at atmospheric pressure, at 170 °C at 7 bar g, and at 90 °C at 0.7 bar a. [1]

Give the supplier the condensate flow rate, temperature and receiver arrangement, the lift and back-pressure to the return point, the available electricity or motive steam or air pressure, hazardous-area or wet-location constraints, packaged receiver requirements, and materials.

02

Choose an electric or a mechanical pump

Spirax describes a mechanical (positive displacement) condensate pump as a body that fills by gravity, with a float that operates changeover valves. When the float rises to a set level, a vent valve closes and an inlet valve opens to let steam or air pressurise the body and push the condensate out; as the level falls the valves change back. It needs no extra control system and runs only when needed. Spirax suggests mechanical pumps where maintenance is an issue, where the environment is hazardous, humid or wet, where electricity is not at hand, or where condensate must be removed from temperature-controlled equipment that may stall. [1]

Because a mechanical pump cycles, Spirax notes that its instantaneous discharge rate can be up to six times the filling rate, and that this figure, not the average flow, must be used to size the discharge pipe. Sizing needs the maximum condensate flow reaching the receiver and the available motive steam or air pressure. [1]

For air-conditioning drainage, Chinafore and Wipcool list compact condensate pumps such as mini pumps and tank pumps with drain accessories, and Zhejiang Feiyue lists condensate pumps among HVAC tools. Guangzhou Xinheng lists automatic condensate pumps among its industrial pumps. [2] [3] [4] [5]

Editorial calculation: a mechanical pump filled at 1,000 kg/h may discharge at up to 6,000 kg/h while it is emptying.

03

Put a value on returning condensate

The US Department of Energy's Steam Tip Sheet #8 recommends maximising hot condensate return to the boiler: returned condensate at 130–225 °F reduces the cold makeup water (50–60 °F) to be heated, cuts makeup water, treatment chemicals and sewer disposal, and high-purity condensate reduces blowdown losses. It states that condensate can hold more than 10% of the steam's energy and gives heat remaining in condensate as (hc − hm) / (hs − hm): for 180 °F condensate (148 Btu/lb), 55 °F makeup (23 Btu/lb) and 100 psig steam (1,189 Btu/lb), 11.0%. [6]

Its worked example returns an additional 10,000 lb/h of condensate at 180 °F for 8,000 hours a year, at 80% boiler efficiency, $8.00/MMBtu fuel, $0.004/gal combined water, sewage and treatment cost and an assumed 12% flash steam loss, giving $33,760 water, sewage and chemical savings and $88,000 fuel savings, $121,760 a year in total. It also reports a specialty paper plant that cut boiler makeup from about 35% of steam production to 14–20% by returning more condensate, saving more than $300,000 a year, and recommends repairing return leaks and insulating return piping. [6]

Editorial calculation: the sheet's figures give 125 / 1,166 ≈ 10.7%, which it rounds to 11.0%. 10,000 lb/h is about 4,536 kg/h, or roughly 4.5 t/h of condensate; 180 °F is about 82 °C and 55 °F about 13 °C. If that flow were returned by a mechanical pump whose discharge can reach six times the filling rate, the discharge line would be sized for up to about 27 t/h. Use your own fuel, water and operating-hour figures. [6] [1]

Give the condensate flow and temperature now lost to drain, operating hours, fuel and water costs, boiler efficiency and return distance and lift, so the supplier can size the pump and you can judge payback.

04

Duty, temperature limit, drive and discharge rate

The discharge flow row applies only to mechanical pumps, which empty at up to six times their filling rate; the line is sized for that burst, not the average. [1]

05

Send the duty, flow and lift data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Duty: [steam condensate; HVAC drainage]
Flow: [kg/h or L/h]
Temperature: [°C]
Lift: [height; back-pressure]
Drive: [electric; steam; compressed air]
Site: [hazardous; wet; electricity available]
Package: [receiver; controls]

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