These designs sometimes reveal that heat transfer packages are a better option than piecemealing a system together — one component at a time — from a variety of different suppliers.
When a heat exchanger stalls, condensate floods the steam space and causes a variety of problems within the exchanger, including the following:
- Control hunting: As condensate backs up in the exchanger, the heat transfer rate to the process is greatly reduced. The control valve opens wide enough to allow flow into the exchanger.
As condensate drains out, the steam space is now greater and the steam pressure increases. The process overheats, the control valve closes down, and the cycle repeats.
- Temperature shock: Conden-sate backed up inside the steam space cools the tubes that carry the process fluid. When this subcooled condensate is suddenly replaced by hot steam due to poor steam trap operations, the expansion and contraction of the tubes stress the tube joints. Constantly repeating this cycle causes premature failure.
- Corrosion from:
Flooding — A flooded heat exchanger will permit the oxygen to dissolve, as well as carbon dioxide and other gases found in the steam. Because the condensate is often subcooled due to the time it is in the exchanger, these gases are more readily dissolved. Together, the cool condensate and dissolved gases are extremely corrosive and will tend to decrease the efficiency of the heat exchanger and reduce the heat transfer through the tubes.
Steam collapse — Under very low loads with the steam valve closed, the steam volume collapses to smaller volume condensate, inducing a vacuum. When the vacuum breaker opens, atmospheric air and condensate mix inside the exchanger, increasing the possibility of corrosion of the tubes, shells, tube sheet, and tube supports.
Freezing — Steam/air coils cannot afford poor condensate drainage, especially if the coil experiences air below freezing temperature. Condensate backed up inside the coil will freeze, often within seconds, depending on the air temperature. A low-temperature detection thermostat is recommended on the coil-leaving side to sense freezing conditions.
System design to reduce stallAs we previously explained, the only way to avoid stall is to eliminate back pressure on the steam trap. There are a number of options available for designing a system that will greatly reduce the risk of stall. The following are two such options:
- Install the heat exchanger in a position so that the condensate will freely drain by gravity to the condensate return line. In many cases this is not possible because of existing piping around the area in which the heat exchanger is needed (e.g., the heat exchanger is installed at a level lower than the condensate return tank).
- Use an electric or pressure-driven condensate pump package installed below the steam trap to pump condensate back to the boiler.
In actual practice, the first option may not be possible, and so the use of electric or pressure-driven pumps to return condensate to the boiler room should be considered.
Figure 1 illustrates a typical piping system for a steam heat exchanger draining into an electric condensate pump package. In this scenario, the steam trap is allowed to operate over the entire load profile, as there is no back pressure on the steam trap.
As steam enters the exchanger, there will always be a positive pressure on the inlet side of the trap, allowing condensate to flow through to the pump receiver. If a vacuum should occur inside the exchanger, then the vacuum breaker will eliminate the vacuum by introducing atmospheric air into the exchanger. With the steam trap installed 15 in. below the exchanger, this will give a 1/2-psi hydraulic head before the trap, creating the differential pressure necessary to drain condensate from the exchanger.
Electric condensate pumps offer a number of advantages that often make them the preferred choice among contractors:
- Contractors are familiar with their operation and ease of maintenance;
- There are many different options available to meet custom requirements;
- Iron or steel receivers are available;
- Relatively small units with low profile equate to smaller required installation space;
- A wide range of sizes are available, catering to diverse applications;
- They can deliver large capacities over wide ranges of back pressures; and
- They can accept condensate flow during the pumping cycle.
Pressure-driven pumps, open-system returnAnother option to ensure proper condensate removal is the pressure-operated pumpless condensate unit. This type of pump uses steam or compressed gas (air) as a “motive force” to create the differential pressure needed to transfer the condensate. There are no impellers, seals, or electric motors used with a pumpless condensate unit.
The unit uses a float mechanism to operate the pump cycle. This float is mechanically connected to an exhaust valve (normally open) and a steam/air valve (normally closed). Condensate flows by gravity through the inlet check valve into the pump body. Air vents through the exhaust valve, allowing the pump to fill.
As the pump fills, the float rises. At a predetermined level, the float mechanism opens the steam valve and closes the exhaust valve, introducing steam or air into the pump. As the pump pressurizes, the inlet check valve is forced closed, and the condensate is pushed out of the outlet check valve and into the condensate return piping system. When the pump empties, the float switches the position of the steam and exhaust valves, and the cycle begins once more.
The pumpless condensate unit does not function as a steam trap; the pump cannot discharge condensate while holding back steam. The ability of the pump to operate and the capacity it can handle are functions of:
- The differential pressure between the motive force and the pressure in the condensate return piping.
- The check valve sizes.
- The height difference between pump and receiver.
The operation allows the pump to accept condensate during the filling cycle only. As the pump discharges, condensate must be stored elsewhere until the discharge stroke is complete. For this reason, a receiver must be installed before the pump. The receiver should be sized for the amount of condensate that will form during the pump’s discharge cycle.
It will store the condensate, eliminating the possibility of backing condensate into the steam space of the heat exchanger. The vent line on the receiver must be sized to handle the flash steam created as hot condensate discharges from the steam trap(s) into the receiver. An undersized vent line will create pressure inside the receiver, raising back pressure against the trap.
The height of the receiver relative to the pump is important. The pump fills up with condensate fed from the receiver. A minimum distance of 12 in. from the bottom of the receiver to the top of the pump is required to provide the pump’s rated capacity.
At distances greater than 12 in., the capacity of the pump increases because the rate at which condensate flows from the receiver to the pump is a function of static head.
The size of the inlet and outlet check valves also has an influence on the pump’s capacity. Standard options are l by l in., 2 by 2 in., and 3 by 2 in. (inlet size by outlet size). The larger the valves, the higher the capacity.
Differential pressure between the motive force and the return line has the largest influence on pump capacity. The larger the differential pressure, the higher the capacity. The pumpless condensate unit operates similar to a steam trap with regard to differential pressure and capacity. The larger the difference between motive pressure and system back pressure, the higher the capacity of the pump.
'Pipe-in' systemFigure 2 (page 96) illustrates a typical pipe-in of a pumpless condensate unit and a steam heat exchanger. The unit is sized to handle the condensing rate of the heat exchanger.
The maximum motive pressure recommended is 125 psi (100 psi is usually more than enough and also eliminates the possibility of water hammer inside the pump that is sometimes present with a 125-psi motive force).
A steam trap is installed at the inlet of the motive force to remove condensate that forms while idle steam condenses in the line during the filling cycle of the pump. The vent line from the condensate unit is tied into the vent line of the receiver. An overflow pipe, installed below this tie-in point, allows condensate to drain should the receiver be undersized or if the pump fails to cycle.
This overflow connection is important in order to prevent condensate from backing up into the steam space of the exchanger, and to warn the operator that possible problems exist in the system.
A pressure gauge should be installed on top of the condensate unit to help diagnose any problems, and to help estimate condensate flow and back pressure. This gauge assembly should include these components:
- Pigtail siphon (used to prevent live steam from entering the bourdon tube which could cause damage to the gauge);
- Isolating valve (normally closed), limiting wear and tear on the gauge when the operator is not using it; and
- Pressure snubber, used to dampen pressure shocks experienced with the sudden introduction of motive force.
During the fill cycle, the pump vents to atmosphere and the gauge should read “0.” As the pump cycles, the gauge will read a pressure reflecting the return pipe system pressure. The gauge is also useful to display the cycle’s length and frequency, as well as the back pressure on the system.