Condensate Pump Head Calculation
Violet Mcdow
Often its necessary to pump condensate generated in heat exchangers and other consumers widely distributed in a plant, back to the condensate receiver in the boiler house. A special challenge with hot condensate, which is often close to 212oF (100oC), is cavitation of the pump and the pump impeller.
Centrifugal pumps generates lower pressure behind the wheels and the hot condensate temporarily evaporates and expands on the back side of the vanes - before it implodes and condensates. Over time this erodes and destroys the pump impeller.
If the absolute pressure exceeds the vapor pressure at the actual temperature of the fluid entering the pump, then the Net Positive Suction Head (NPSH) is positive and its possible to avoid cavitation.
If it's not possible to increase the suction pipe and lowering the pump under the receiver it may be possible to reduce the absolute evaporation pressure in the condensate Pvp by reducing the condensate temperature with a cooling exchanger in the suction pipe.
A pressure powered pump use steam or air pressure to push the condensate from the receiver back to the boiler room. In principle its a simple intermittent mechanical construction working in cycle where a receiver in the pump is filled with condensate before steam or pressurized air pushes the condensate out and back to the boiler room.
The pumping of other boiling liquids - like LPG (-43oC in normal atmospheric pressure) - offers the same challenges to manufactures and users. LPG is stored at exactly its boiling point (at the actual pressure in the tank) and any increase of temperature, as well as any decrease in pressure, will cause the product to boil and form vapor. In many installations, the suction friction head is equal or larger than the static suction head, making the available NPSH a negative value. The pressure drop due to the flow restrictions in the inlet piping system, e.g., excess flow valve, control valves, fittings, strainer, etc., will induce the LPG vapor formation at the pumps suction port.
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A condensate pump is a specific type of pump used to automatically collect, move and direct the flow of HVAC condensate when gravity drainage is not possible or practical. If not properly collected and removed for HVAC equipment, condensate can damage buildings or property, including the growth of bacteria, mold, and algae, along with unpleasant odors.
Condensation (water) is commonly produced by HVAC equipment (heating and cooling), air conditioning systems, high-efficiency furnaces, condensing boilers, high-efficiency instant water heaters, dehumidifiers, steam humidifiers, refrigeration equipment, and others.
Condensate pumps are typically sized at three times (3x) the normal condensing or evaporation rate for your appliance (i.e. HVAC system). These guidelines are based on design and operating experience from HVAC professionals. However, upon proper application, units can be sized smaller (1.5-2x) depending on the condensing or evaporation rate of your HVAC equipment and the climate.
Although some of the concepts are the same, sizing small fractional horsepower condensate pumps is different from sizing a large industrial pump used for commercial, industrial, municipal, or agricultural applications. Below we explain more about lift, head, friction loss, and other factors as they relate to condensate pumps used in residential and light commercial applications.
Vertical Lift, also known as Static Head, Pump Head, or Lift, is the height that condensate water must travel as it moves through your discharge hose/pipe. To estimate this, measure the vertical distance from the condensate pump outlet to where the discharge pipe turns from vertical to horizontal.
As water flows through your discharge hose/pipe, friction is created from water rubbing against the inside of the hose/pipe. This restricts the water flow, and your condensate pump must overcome this friction.
If you think friction could be an issue for your condensate pump system, consult a certified plumber to find out how much friction is created in your discharge hose/piping. They can give you a Friction Head estimate, which you then add to the Vertical Lift (Static Head) to arrive at a Total Dynamic Head (e.g. Lift) that your condensate pump must be capable of producing.
In applications where installation space is limited, a small, low profile or in pan condensate may be required. Make sure you consider where the pump will be mounted and if there are any space constraints to consider.
It should not be assumed that the drain line (and trap) should be the same size as the plant outlet connection. The plant may operate at a number of different operating pressures and flowrates, especially when it is temperature controlled. However, once the trap has been correctly sized, it is usually the case that the drain line will be the same size as the trap inlet connection, (see Figure 14.3.1).
Regarding the conditions inside the drain line, as there is no significant pressure drop between the plant and the trap, no flash steam is present in the pipe, and it can be sized to carry condensate only.
For practical purposes, where the drain line is less than 10 m, it can be the same pipe size as the steam trap selected for the application. Drain lines less than 10 m long can also be checked against Appendix 14.3.1 and a pipe size should be selected which results in a pressure loss at maximum flowrate of not more than 200 Pa per metre length, and a velocity not greater than 1.5 m/s. Table 14.3.2 is an extract from Appendix 14.3.1.
The section of pipeline downstream of the trap will carry both condensate and flash steam at the same pressure and temperature. This is referred to as two-phase flow, and the mixture of liquid and vapour will have the characteristics of both steam and water in proportion to how much of each is present. Consider the following example.
From this, it follows that the two-phase fluid in the trap discharge line will have much more in common with steam than water, and it is sensible to size on reasonable steam velocities rather than use the relatively small volume of condensate as the basis for calculation. If lines are undersized, the flash steam velocity and backpressure will increase, which can cause waterhammer, reduce the trap capacity, and flood the process.
Steam lines are sized with attention to maximum velocities. Dry saturated steam should travel no faster than 40 m/s. Wet steam should travel somewhat slower (15 to 20 m/s) as it carries moisture which can otherwise have an erosive and damaging effect on fittings and valves.
Condensate discharge lines from traps are notoriously more difficult to size than steam lines due to the two-phase flow characteristic. In practice, it is impossible (and often unnecessary) to determine the exact condition of the fluid inside the pipe.
Because of the number of variables, an exact calculation of line size would be complex and probably inaccurate. Experience has shown that if trap discharge lines are sized on flash steam velocities of 15 to 20 m/s, and certain recommendations are adhered to, few problems will arise.
2. If it is unavoidable, non-pumped rising lines (Figure 14.3.4) should be kept as short as possible and fitted with a non-return valve to stop condensate falling back down to the trap. Risers should discharge into the top of overhead return lines. This stops condensate draining back into the riser from the return main after the trap has discharged, to assist the easy passage of flash steam up the riser.
Important: A rising line should only be used where the process steam pressure is guaranteed to be higher than the condensate backpressure at the trap outlet. If not, the process will waterlog unless a pumping trap or pump-trap combination is used to provide proper drainage against the backpressure.
3. Common return lines should also slope down and be non-flooded (Figure 14.3.4). To avoid flash steam occurring in long return lines, hot condensate from trap discharge lines should drain into vented receivers (or flash vessels where appropriate), from where it can be pumped on to its final destination, via a flooded line at a lower temperature.
Establish the point where the steam and condensate pressures meet (lower part of the chart, Figure 14.3.5). From this point, move vertically up to the upper chart to meet the required condensate rate. If the discharge line is falling (non-flooded) and the selection is on or between lines, choose the lower line size. If the discharge line is rising, and therefore likely to be flooded, choose the upper line size.
Note: The reasoning employed for the sizing of a steam trap is different to that used for a discharge line, and it is perfectly normal for a trap discharge line to be sized different to the trap it is serving. However, when the trap is correctly sized, the usual ancillary equipment associated with a steam trap station, such as isolation valves, strainer, trap testing chamber, and check valve, can be the same size as the trapping device selected, whatever the discharge line size.
Because the condensate will have lost its flash steam content to atmosphere via the receiver vent, the pump will only be pumping liquid condensate. In this instance, it is only necessary to use the top part of the chart in Figure 14.3.5. As the line from the pump is rising, the upper figure of 25 mm is chosen.
The common line slopes down to the flash vessel at 1.5 bar g, situated in the same plant room. Condensate in the flash vessel falls via a float trap down to a vented receiver, from where it is pumped directly to the boiler house.
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