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Quotes - Engineering - SalesThe loop as a system
Where the heat goes, and where it comes back
A hot WFI system is a tank, a pump, a loop and a heater. The still or membrane unit fills the tank; the pump circulates the loop past every use point and back at 80 C or above; the heater replaces what the tank walls, the pipe, the use-point coolers and the sub-loops take out. What is hard is the accounting: the heater is sized on losses nobody has measured, and the sub-loop coolers on draws that happen ten minutes a shift.
This page covers that accounting, the cold sub-loop, the condensate leg and the return temperature control.
Loop heater, sub-loop cooler, condensate and control
The loop heater is sized on losses, not on the tank
Once the system is hot the heater sees only what the loop loses. A 10,000 gallon insulated tank at 82 C in a 22 C room loses 20,000 to 40,000 Btu/h; 800 ft of 2 in insulated pipe loses about the same; each use-point draw takes out 80 C water and returns nothing. On a mid-size system the steady loss is 60,000 to 150,000 Btu/h, and a 200,000 Btu/h heater holds it with margin. The cold start from ambient is done by the still filling the tank hot, not by the loop heater. Sizing the heater for a two-hour cold start it will never do gives a unit that cycles its steam valve all day and hunts the return.
The heater is a vertical double tubesheet shell-and-tube with the loop on the tube side and steam at 15 to 30 psig on the shell, a shell temperature of 121 to 134 C: enough driving force for a compact unit and low enough that a stuck valve cannot flash the loop.
Cold sub-loops for suites that need 20 C water
A formulation suite that draws WFI at 20 to 25 C for an hour at a time is badly served by a use-point cooler that starts and stops on each draw. A cold sub-loop is the alternative: a branch off the hot loop through a sub-loop cooler, circulated cold past several use points, and returned to the tank. The cooler sees the full sub-loop flow while the suite runs, 82 C to 22 C on chilled water, and is sized on that flow. When the suite finishes, the sub-loop is drained and steamed, or returned to the hot loop and reheated by the main heater, which is the other reason the heater carries margin.
Condensate, traps and the air gap
Steam condensate from the heater shell is plant condensate and goes back to the boiler through a trap at the shell low point. It never runs to a floor drain near the WFI tank vent or a use point, and it never shares a header with a sanitary drain. The trap sits below the shell so the shell drains dry when steam is off, because a shell holding 100 C condensate against 80 C tubes is a thermal shock at the next start. An air gap between the condensate line and any drain is the rule; a direct connection is a cross-connection finding.
Return temperature control and the tank
The control loop is simple: a transmitter on the return to tank, a modulating steam valve on the heater, a setpoint of 82 to 85 C and a low alarm at 78 C. The heater outlet is not the control point, because the return is the coldest place in the loop and the SOP names the return. A well-insulated tank with a loop heater needs no jacket; what it needs is a heated vent filter and a spray ball fed from the loop so the headspace and walls above the water line are wetted at 80 C, because that is where a hot system grows organisms if it grows them anywhere.
Send HeatX the tank size and insulation, loop length and pipe size, the use points and the steam pressure, and the heater and coolers are sized as a set. Call and talk it through with an engineer: 1-805-484-2992
Hot WFI distribution process conditions
This table describes a heater and sub-loop coolers sized to keep a storage loop hot and alarmed. Check it against your tank, return temperature and steam supply, then send those figures with your quote request.
| Condition | Detail |
| Product | WFI in a recirculating loop; 1,000 to 20,000 gallons stored |
| Flow range | 30 to 400 gpm loop; 5 to 60 gpm on a cold sub-loop |
| Temperatures in / out | Heater 78 to 82 C in, 82 to 85 C out; sub-loop cooler 82 C to 20 to 25 C |
| Hold | 80 C minimum at the return, continuously, alarmed at 78 C |
| Utility | Plant steam 15 to 30 psig; chilled water 7 C on sub-loop coolers |
| Approach | Heater 15 to 25 C; cooler 3 to 5 C |
| Construction | Double tubesheet straight tube, vertical, steam on the shell |
| Finish / class | 316L, 15 to 20 Ra electropolished; ASME BPE; drains to one low point |
| Condensate | Steam trap at the shell low point, to plant return through an air gap, never to a drain near WFI |
| CIP / SIP | Derouging with the loop; a cold sub-loop is steamed at 121 C after each use |
Common FAQs
A loop heater holds the coldest point in the system, the return, and keeps steam equipment off the tank.
3 ft/s or more at the return under all draw conditions; the pump is sized with every use point closed and checked with the largest draw open.
Yes, if it is run as an ambient loop with its own sanitization schedule. Most sites return it hot between campaigns.
It rarely sees 121 C, but a 135 C SIP rating costs little and lets the site steam the loop after an excursion.
Related
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