Condensate Considerations: A Pump Primer
In most systems, the condensate is treated with additives to reduce corrosion. If condensate is dumped down the drain and fresh makeup water is added, additional chemicals would be required to condition the water. Dumping hot condensate is thermal pollution and the temperature at which the water may be dumped into the drain is regulated. Also, fresh water is not free. Every gallon of condensate dumped into the drain must be replaced with one gallon of fresh water. For these reasons, condensate should be captured and reused.
BasicsIn a simple, one-pipe steam system, energy is added to water in the boiler to create steam. The steam flows through the interconnecting piping to the radiation where it gives up its latent heat and condenses back into water (condensate). The condensate flows back to the boiler through condensate return piping where it repeats the cycle over. The steam piping is designed for very low-pressure losses, in the range of ounces. This enables the boiler to operate at low pressure. The condensate piping is pitched back to the boiler so the condensate is returned by gravity. The condensate flows back into the boiler when the weight of a vertical column of condensate exceeds the pressure loss of the steam piping. The static height of condensate is the "A" dimension shown in Figure 1. The height of the vertical column of condensate is 28 in. for each 1 psig boiler pressure. This simple system required no boiler feed pump to replenish the boiler with condensate.
The simple steam system did not remain simple. In time, the steam and condensate were separated into different lines. The one-pipe steam systems became two-pipe systems. Buildings and systems became bigger. Horizontal runs of return piping became longer. As a result, the installations lacked the vertical column height of water needed to push the condensate back into the boiler.
Selecting Condensate EquipmentThe selection of condensate-handling equipment should be given careful consideration to maintain a properly balanced, efficient steam system. The factors to consider when selecting condensate return equipment are:
- The system size;
- The required discharge pressure;
- The NPSHA of the system (temperature of the condensate);
- The amount of makeup required due to leakage;
- Flash steam and steam consumed in industrial processes; and
- The change in load rate during various time periods.
The condensate return rate can be calculated from the sq ft equivalent direct radiation (EDR) served, the lbs/hr of steam used, the Btu heat load, or the boiler (bhp) required. This information will enable you to select the size of the holding tank or receiver and the required volume of condensate the pump must move.
Determine Pumping CapacityEDR, by definition, is the amount of heating surface that will give off 240 Btuh when filled with a heating medium at 215?F and surrounded by air at 70?. Steam is the medium in the radiation. When steam condenses in the radiation, it gives up its latent heat then flows back as condensate. The latent heat of vaporization of 5 psig saturated steam is 960 Btu/lb. Using the conversion factor: 1 EDR = .25 lbs/hr. (condensate).
Knowing the sq ft of heating surface (EDR) the condensate unit will be serving, we can calculate the rate that steam will condense. Factoring in the density of water at the condensing temperature, we can convert the lbs/hr of water to a more common volumetric flow rate, gallons per minute.
62.4 lb x 1 ft3 = 8.34 lb 1 ft3 7.48 gal gal
and X lb x 1 gal x 1 hr = Y gal hr 8.34 lb 60 min min
or X lb x 1 = Y gal hr 500 min
But at 200?, density is 60.1, and the factor is 482; at 300 ?, density is 57.3, and the factor is 460.
We have standardized on the conservative factor of 500:
Using the formula lbs/hr ? 500 = gpm,1 EDR requires .0005 gpm of steam condensing or 1,000 EDR = .5 gpm.
Converting the flow to gallons per minute simplifies pump and tank selection. The tank or receiver is sized for 1 to 5 min net storage capacity based on the return rate. Condensate pumps are typically sized for twice the condensate return rate (see sidebar).
Determine the Required Discharge PressureThe pump must be sized to meet the total dynamic head required by the application. It must be sized to:
- Overcome any pressure differences;
- Add static head to lift the condensate;
- Overcome losses in the return piping, fittings, and valves; and
- Once you’ve summed these terms, add an additional 5 psig if the total pressure is 50 psig. If the calculated sum is above 50 psig, add 10 psig. This allows for wear in the system.
Determine the NPSHAAs with all pump applications, NPSH must be considered. When selecting pumps for condensate, this is especially critical. The amount of required head is a function of the pump design and is called Net Positive Suction Head Required (NPSHR). NPSHR is the amount of suction head required to prevent pump cavitation and is indicated on the pump curve. The NPSHR for a 609PF pump is illustrated in the curve on Figure 2 labeled NPSH REQ.
If the pressure in the pump drops below the vapor pressure of the condensate, cavitation or flashing will occur. Each application has an available net positive suction head (NPSHA). NPSHA is a function of the static head at the pump suction, velocity head, and vapor pressure of the liquid at the temperature being pumped. The NPSHA must always be greater than the NPSHR or noise and cavitation will occur. The formula for NPSHA is:
NPSHA = 2.31 (Pa-Pv) + (He – Hf) spgr Where: Pa: pressure in the receiver, psia. Pv: vapor pressure of the liquid at its maximum temperature, psia. He: elevation head, feet. Hf: friction losses in the suction piping at the required flow rate, feet.
In a vented receiver, Pa is atmospheric. He is the height of condensate above the pump suction, determined by the condensate unit design. Hf is the friction losses from the receiver to the pump suction. This is fixed by the condensate unit design. Pv is the vapor pressure of the liquid at the pumping temperature. For a particular, previously designed condensate return unit, the only variable is Pv. Since Pv is a function of temperature, once the condensate temperature is known, we can determine NPSHA.
The Hydraulic Institute recommends a margin ratio between the available and required NPSH (NPSHA/NPSHR) for various centrifugal pump applications. Using building services as the application category for boiler feed pumps or condensate return pumps, the ratio margin suggested by the Hydraulic Institute is 1.1 ft or 2 ft minimum, which was developed using field experience from many pump manufacturers as the basis.
Condensate units are typically pre-engineered to ensure NPSHA is greater than NPSHR for a given temperature limit. Most manufacturers catalog their condensate return equipment by maximum temperature. (There is a correction factor for increased elevation. Boiling point decreases 1? for every 500 ft increase in elevation.) Condensate return units are primarily used to transport condensate from the far reaches of a steam system back to the boiler room when gravity flow is no longer feasible. In any system, there is a time lag between when steam leaves the boiler until it returns in the form of condensate.
The greatest time lag exists during a cold start-up. In a cold system, the steam mains, radiators, and return piping are completely drained. When the system is put into operation, the steam mains and radiators require a volume of steam to fill the system. The volume of steam must come from the boiler during this period and causes a drop of water level in the boiler.
Additional time is required for the condensate to flow through the return lines back to the boiler room either by gravity or to be pumped back from condensate transfer units. The opposite condition occurs when the system is shut down; all the steam in the mains and radiators is returned in the form of condensate and must be stored for the next system start-up.
Boiler Feed UnitsThe practical system size limit for a condensate return unit feeding a boiler is approximately 8,000-sq-ft EDR or 60-bhp system size. The maximum system size will vary with different types of boilers having various storage capacities between high and low operating levels.
Systems larger than 8,000-sq-ft EDR use a boiler feed unit to supply condensate to the boiler. The boiler feed unit consists of a storage receiver sized to store an adequate volume of water to handle the system surges or system time lag. The pumps are started and stopped to maintain the boiler water level in the boiler. The boiler feed receiver must be sized large enough to prevent overflowing of condensate during system surges.
The normal receiver sizing is to provide 5 min storage volume for systems up to 30,000-sq-ft EDR or approximately 200 bhp. Larger systems should provide a minimum of 10 min storage volume. Large, single-story buildings requiring over 100,000-sq-ft EDR and campus complexes should provide a minimum of 15 min storage volume.
Undersizing the boiler feed receiver can cause an overflow of returned condensate, which must be replaced with fresh makeup water. This wastes heat, makeup water, and chemical treatment. Once you have decided a boiler feed unit is needed for the system, the following information must be determined.
- Determine the unit load requirements;
- Select the type of control system required;
- Calculate the pump capacity;
- Calculate the pump discharge pressure;
- Select the basic unit; and
- Select the desired accessories.
The load requirement is based on the boiler capacity not the system capacity. The boiler is normally rated in bhp. It may also be rated in lbs/hr, sq ft EDR, or Btu output. Convert this data to the boiler steaming rate, then convert this to gal/min using the relationships detailed earlier.
Select the Type of Control System RequiredThere are several choices in type of piping arrangements to meet various requirements. A simple arrangement is one (or two) pumps feeding one boiler. When the water level in the boiler drops, the pump control on the boiler activates the pump. When the water level in the boiler reaches the correct level, the pump control deactivates the pump.
If the pump control is a single level and a second boiler feed pump is included on the unit, the second pump can be manually operated as a standby. If the pump control can detect two separate levels, the second pump can be automatically activated as a standby if the water level in the boiler drops to the second level.
Another option is automatic alternation, which automatically switches the lead pump after every activation cycle. This ensures even wear on each pump and prevents the lag pump from sitting idle for long periods (Figure 3).
One duplex boiler feed unit can feed two boilers. This arrangement has a dedicated pump for a dedicated boiler. When either boiler requires water, the pump dedicated to that specific boiler will be activated. The two pumps can be manually switched to the other boiler by switching the pumps on the selector switches and manually closing and opening the valves in condensate piping. If the boilers are equipped with two-level pump controllers and boiler feed valves are installed between the pumps and boilers, the standby pumps can be automatically activated should the water level in the boiler recede to the second level (Figure 4).
A third arrangement is two boilers being fed by three pumps. Each boiler has a dedicated pump. The third pump is a dedicated manual standby. This arrangement can be expanded by adding one boiler feed pump for each additional boiler. Similar to the example above, if the boilers are equipped with two-level pump controllers and boiler feed valves are installed between the pumps and boilers, the standby pumps can be automatically activated should the water level in the boilers drop below the second level (Figure 5).
These control system examples illustrate three common (but not the only) methods of linking the boiler feed unit to the boiler. Unlike a condensate unit, a boiler feed unit relies on external signals for pump control. When selecting the boiler feed unit, the number of boilers being fed, the number of pump control levels being monitored, whether boiler feed valves are used, the type of standby, and the type of alternation are all system-dependent criteria that will affect the design of the boiler feed unit.
Calculate the Pump CapacityThe individual pump’s capacity should be based on the boiler(s) they are required to feed. The pumps are normally sized for 1.5 to 3 times the boiler load. When boiler feed pumps are selected for continuous operation, they are normally sized for 1.5 times the boiler load. Boiler feed pumps for intermittent operation are normally sized for twice the boiler load. When there is a danger of pump cavitation or when turbine pumps are used, selecting the pumps at three times the boiler load is accepted.
Calculate the Pump Discharge PressureThe required discharge pressure for a boiler feed unit is calculated similar to a condensate unit. It is the sum of static head lift, friction losses in the piping, and the pressure in the boiler. In addition to the boiler pressure, a safety margin of 5 or 10 psig is added. For boiler pressures up to 50 psig, add 5 psig. For boiler pressures of 51 psig or higher, add 10 psig for safety.
Select the Basic UnitWe now know the required tank size, control system, and pump duty point. Next we need to decide the type of unit. Things to consider are condensate temperature, materials of construction, and receiver location (underground, floor mounted, or elevated). Most manufacturers have preselected packages of receivers and pumps to meet a specific boiler load. These preselected packages are stated in EDR served or bhp.
Select the Desired AccessoriesOnce the basic unit is determined, the options can be selected. Typical options include gauge glass, thermometers, discharge pressure gauges, low-water cutoff switch, and alarm switch.
A gauge glass, thermometer, and discharge pressure gauge are highly recommended options. They simplify installation and help monitor the system. They are used to troubleshoot system problems and are well worth the investment.
Pumps Designed for Handling Hot CondensateCertain characteristics make a pump more desirable for handling hot condensate. Centrifugal and turbine pumps are commonly used for condensate handling.
A condensate pump should have a low NPSHR, a low sensitivity to sediment and corrosion, and a low inertia for frequent starts and stops. It should also have the ability to start after long periods of inactivity, along with generous running clearances to retain original capacity after years of service.
Sediment and corrosion are prevalent in condensate return systems. Condensate return units are designed with a low sensitivity to both. Large strainers are installed on the receivers keeping large particles out of the receiver. Typically the strainers are designed for easy cleaning with large dirt pockets to minimize maintenance. The pumps have generous running clearances and retain near their original capacity after years of service.
Condensate pumps are selected to return condensate at twice the system condensing rate. If a system sends condensate back at the rate of 10 gpm, the pump is sized to return it at 20 gpm. This enables the pump to return the condensate quickly and allows the pump to keep up during peak periods and start-up. Pumps that operate at a higher speed (3,500 rpm) for a given duty point produce lower inertia loads on the pump shaft when compared to low-speed (1,750 rpm) pumps. However, there is a trade off: Higher speeds produce more noise and require higher NPSHR. For most applications, the high-speed pump is preferred for lower cost and lower inertia loads.
Many condensate transfer units operate seasonally with long periods of inactivity. These units may sit unused for months. Pump materials are selected to reduce corrosion between the impeller and volute. Bronze wear rings are installed in the volute to minimize galvanic reaction.
Condensate transfer and boiler feed units are specifically designed for this duty. Properly selected and installed, these units will provide you with many years of trouble-free service. ES
ReferencesAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers, 1996 ASHRAE Handbook, HVAC Systems and Equipment. I-P Edition, pp. 10.3-10.5.
ITT Fluid Handling Training and Education, Domestic Pump Product Application and Sizing Manual, Bulletin No. TES-676, Revision 1, pp. 4-10, 32-40.
ITT Fluid Handling Training and Education, Hoffman Steam Heating Systems, Design Manual and Engineering Data, Bulletin No. TES-181, pp. 7.
Hydraulic Institute, American National Standard for Centrifugal and Vertical Pumps for NPSH Margin, ANSI/HI 9.6.1-1998, pp. 5.
Conversion Factors1 bhp = 140 sq ft EDR or 33,475 Btuh, or 34.5 lb/hr steam at 212?F.
1,000 sq ft yields .5 gpm condensate.
To convert sq ft EDR to lb of condensate: divide sq ft by 4.
.25 lb/hr condensate = 1 sq ft EDR.
1 sq ft EDR (steam) = 240 Btuh with 2158 steam filling radiator and 708 air surrounding radiator.
Size condensate receivers for 1 min net storage capacity based on return rate.
Size boiler feed receivers:
- 5 min net storage for systems up to 200 bhp;
- 10 min net storage for systems above 200 bhp; and
- 15 min net storage for systems that exceed 100,000 sq ft EDR or 700 bhp.
Size condensate pumps at twice the condensate return rate.
Size boiler feed pumps at twice boiler evaporation rate or .14 gpm/boiler hp (continuous running boiler pumps may be sized at 11/2 times boiler evaporation rate or .104 gpm/bhp. When there is a danger of pump cavitation, or when turbine pumps are used, select pumps for three times the evaporation rate.