Customer Can't Get Enough Flow through Condensate Polisher => How to Correctly Size a Condensate Polisher
Customer called to state they couldn't get enough flow through their new condensate polisher. During the service visit, it was determined that the Deaerator and Condensate system was not being operated in accordance with Best Practices Procedures presented by the US Department of Energy's Office of Industrial Technologies, Energy Efficiency and Renewable Energy.
In this case the customer was feeding softened make up water into the Condensate tank along with the returned condensate, directing this water through the condensate polisher, then sending to the Deaerator. A Sodium Cycle Ion Exchange Condensate Polisher is typically sized and designed to treat only returned condensate, not to heat up make up water. In this case the polisher needed to handle both the make up and returns. It was never sized for this.
Normally, the softened make up water goes to the inlet of the deaerator, is heated by steam to remove the dissolved gasses, stored in the deaerator storage tank and sent to the boiler. The low-pressure condensate goes to a condensate receiver, through a condensate polisher and sent to the deaerator storage tank.
Most likely, the customer was thinking the condensate return, which is hot, would heat up the make up water before going to the deaerator, thus saving energy and steam. Conventionally, deaerators are designed to take cold softened make up water and heat it up in the steam chamber to remove oxygen and CO2 before sending it to the boiler.
The best way to save energy would be to install a Boiler Blowdown Heat Recovery System. This consists of a Blowdown Flash Tank and Heat Exchanger. Simply put, with this system the boiler blowdown water goes to a Flash Tank, where the pressure is reduced from the boiler operating pressure to the Deaerator operating pressure; typically 5 psig (228 deg F). When this blowdown water pressure is reduced, most of the water flashes to steam. The steam is directed to the deaerator steam inlet. The remaining blowdown water goes to a shell and tube or coil type heat exchanger, which is used to heat the softened make up water prior to going to the deaerator. The heat exchanger also reduces the blowdown water to safe and acceptable temperature levels for drains.
Separate from this, the condensate return water, having traveled through the myriad of piping, contains iron (crud) and some hardness and needs to be "polished" before being recycled back to the boiler. The condensate polishers are designed to handle only the returned condensate, which is a percentage of original make up water. When sizing up a condensate polisher we need to know the percentage of make up water that is "returned". It is only that percentage we are concerned about when we size up the condensate polisher.
Thus, a basic system would look like this:
The better energy saving system (as described above) would look like this:
A condensate polisher is used in addition to this Boiler Blowdown configuration.
Use of a condensate polisher and a good chemical treatment program enables the customer to use less make up water and energy. When considering use of a condensate polisher it's important to size it for treatment of the condensate only.
For more information about energy savings and Boiler Blowdown Systems see attached pdf files that the government has made available via the Internet.