Ahh, you're talking about some fancy labratory stuff. From the article;
"E.Polishing Loop:
In small Lab DI systems most of the water is used immediately or shortly after it is made. There really is no necessity to keep re filtering the water in the containment vessel. In larger systems where you need 50 gallons always available, under pressure for rinsing and always at high quality, it is necessary to include a polishing loop in the system. The polishing loop is a series of resin filters that has the water pumped from the holding vessel though the filters and back into the containment vessel. This keeps the water at a high resistivity rate in the holding vessel. In this type of system a faucet or spray wand can be added in the loop since the water is always under pressure in the pumping loop. This is an ideal system for a small lab that requires 100-200 gallons per day of DI Water. The entire system, including pre filters, sensors, holding tank and monitors can be built into a console that provides a ready source of DI water for the Lab. The larger systems have incorporated additional instrumentation that monitors the quality of the DI water and displays it on a digital Resistivity Meter. A built in alarm also is included
The system on the left is a typical DI system Mounted in a console to feed a lab with 5 stations that require DI Water. The system contains a polishing loop that has plumbing that extends into the individual stations. At each station a final point of use .5 micron absolute filter prevents any particulates from reaching the use point.
This system is entirely mounted on a 5 ft console with all the filters and pumps easily accessible for replacement. The electronics for the system are mounted in the rear of the upper back plane. A resistivity monitor and a TDS monitor control the system and sound an alarm if resistivity drops below a certain level. This system produces up to 180 gallons of DI [18 Meg Ohm] per day and stores another 50 gallons.
The system incorporates a sink and Lab Faucet. The system may also be fitted with a sump if no drain is available nearby. The sump vessel will pump the reject product and any drain water from the sink to a nearby drain. Small ultrasonic tanks can also be installed in the system to give you a complete cleaning system on a cart. Heaters can also be added to the system to provide hot DI on demand. Special filters can be added to the loop system to provide biophage removal for medical applications.
Larger Lab systems up to 500 gallons per day are available; these systems will provide water enough for limited production cleaningof parts. They can also be designed to supply 4-5 stations within a lab to provide water to faucets or other equipment on demand. These Systems can be remote controlled by having the control panel in the lab and the DI unit in another room. The systems can also have water chillers and heaters to maintain a specific tempature of the DI water in the Loop.
Options such as point of use filters at specific stations in the lab give the water a final polish just before use."