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Quotes - Engineering - SalesFixed Tubesheet Condensers
Welded Tubesheets, Lowest Cost per Square Foot
The simplest and least expensive shell and tube configuration per square foot of surface. Where the condensing side is clean and thermal expansion is modest, a fixed tubesheet condenser is very hard to beat on installed cost.
In a fixed tubesheet exchanger the tubesheets are welded directly to the shell and the tubes run straight between them. There are no packed or gasketed joints on the shell side, no floating components and no U-bends. The construction is about as simple as a shell and tube exchanger can be, and the NEN variant — where both the shell and the head are welded to the tubesheet — is the lowest cost of all.
What you gain. Cost, obviously. But also a genuinely leak-tight shell side, because there is nothing on that side to leak through. Straight tubes mean the tube bores can be cleaned mechanically or chemically without difficulty, and a bolted cover plate on the front head makes that access straightforward. For a condensing duty where the vapor is on the tube side and the coolant on the shell side, that combination is often ideal.
What you give up. Two things, and both matter on condensing service. First, the outside of the tubes is inaccessible: the bundle cannot be removed, so shell-side cleaning is chemical only. If the condensing fluid is on the shell side and it fouls, that is a serious constraint. Second, there is no provision for differential thermal expansion. The shell and the tubes are rigidly connected at both ends, so if they want to change length by different amounts, something is carrying that load.
The expansion joint question. Condensing duty often involves a large temperature difference — saturated steam on one side, cold cooling water on the other. That produces exactly the differential expansion a fixed tubesheet cannot absorb, and the answer is a shell-side expansion joint, sometimes combined with rod baffles for tube support. The joint works, but it adds cost and one more component that can fail, and it erodes the cost advantage that made fixed tubesheet attractive in the first place. This is the standard trade-off: once an expansion joint is required, a U-tube bundle may well come out cheaper.
Thermal shock is the real caution. Steady-state temperature difference is predictable and can be engineered around. Rapid transients are harder. Steam cleaning a fixed tubesheet unit that has no expansion joint is generally not recommended, and any condensing service subject to sudden startup, trip or blowdown transients deserves a hard look at whether fixed tubesheet is the right choice at all.
Where a bundle must come out for cleaning, see U-tube and floating head construction. For the shell type decision, see our shell and tube overview.
Configuration
- TEMA Types: BEM, AEM, NEN
- Bundle: Not removable
- Expansion: Joint if required
- Vapor Side: Usually tube side
Features
- Lowest cost per sq ft
- No shell-side joints
- Straight, cleanable tubes
- Bolted cover plate option
- Rod baffle tube support
Typical Condensing Duty
- Clean steam condensing
- Surface condensers
- Vent and flash condensing
- Refrigerant condensing
- Utility condensing duty
| PRODUCT | STYLE | TEMA TYPE | BUNDLE |
| Vapor Condenser | Fixed Tubesheet | BEM / AEM / NEN | Not removable |
Selecting Fixed Tubesheet for Condensing Service
When It Is the Right Answer
Fixed tubesheet construction suits condensing duty where three things are true: the fouling side is the tube side or neither side fouls appreciably, the steady-state temperature difference between shell and tubes is moderate, and the service is not subject to severe thermal transients. Under those conditions it delivers the same thermal performance as a far more expensive configuration and there is no good reason to pay for a removable bundle you will never remove.
Clean steam condensing with the steam in the tubes and cooling water in the shell is a textbook fit. So are many surface condenser, vent condenser and refrigerant condenser duties. The common thread is a clean condensing stream and a coolant that can be managed chemically.
When to Look Elsewhere
If the condensing fluid is on the shell side and carries anything that will deposit — polymer, salts, entrained solids, heavy ends — fixed tubesheet is a poor choice, because you will never get a brush onto that surface. If the temperature difference is large enough to require an expansion joint, price a U-tube alternative before committing, because the cost advantage may already have evaporated. And if the unit will see rapid thermal cycling, the case for U-tube construction is strong on mechanical grounds alone.
Practical Details Worth Specifying
A bolted cover plate rather than a welded bonnet on the front head costs little and makes tube cleaning genuinely practical rather than nominally possible. Rod baffles instead of plate baffles can reduce the stresses that drive the need for an expansion joint while also lowering shell-side pressure drop, which is useful on vacuum condensing. And on any condensing duty, the position of the noncondensable vent connection deserves as much thought as it does in a more expensive exchanger — a cheap condenser blanketed with gas performs no better than an expensive one.
Fixed tubesheet questions
The savings come from parts that are not there: no floating head assembly, no backing ring, no second gasketed closure, and no U-bend forming, so there is fewer machined components and fewer welds to inspect. On a typical condenser the fixed tubesheet price is the baseline, a U-tube unit runs roughly 10 to 20 percent above it, and a floating head unit 30 to 50 percent above it.
The other saving is surface per shell. Straight tubes fill the whole shell length with surface, where a U-bundle loses some length to the bends and a pull-through head gives up a ring of tubes around the floating cover. For a given duty the fixed tubesheet shell is therefore slightly smaller as well as simpler, which compounds the saving on the shell plate, the saddles and the foundation.
Only by circulation, and there is two chemical routes that work on most condenser fouling: an inhibited acid for mineral scale and a hot caustic or solvent wash for organic films. Each needs a connection at the shell top and bottom, a circulation pump sized to move the solution at 1 to 2 ft/s through the bundle, and a drain at the true low point.
Specify those connections when the unit is bought, because cutting them in later is an ASME repair. A shell-side condensing stream that leaves polymer or heavy ends will not dissolve in anything safe to circulate, and that is the duty to keep off a fixed tubesheet. Check the result by comparing shell-side pressure drop and the approach temperature before and after the wash, since solution that short-circuits through the baffle clearances can leave the bundle center untouched.
When the calculated differential growth between shell and tubes would overstress the tube-to-tubesheet joints, and there is only a few numbers in that calculation. Take a 16 ft unit with stainless tubes condensing steam at an average metal temperature of 180 F and a carbon steel shell at 90 F: the tubes want to grow about 0.2 in and the shell about 0.03 in.
That 0.17 in difference over 192 in is a strain of nearly 0.001, which in steel is a stress in the region of 25,000 psi, well past what the joints can carry for long. So the duty needs a joint, a rod baffle arrangement that lowers the load, or a different rear head. Below about 50 F of metal temperature difference the fixed design usually passes without a joint; above 100 F it rarely does, and a U-tube quotation is worth having beside it.
Not without an expansion joint, and even then the rate of change is the thing to control. A tube bundle hit with 300 F steam while the shell sits at 70 F tries to grow its full differential in a minute or two, and the shell has no time to follow.
If the tube bores need cleaning, mechanical brushing through the bolted cover is safer and more effective than steam. If a thermal clean is unavoidable, bring the unit up with hot water first so the shell and bundle climb together, and limit the ramp to a few degrees per minute. The same discipline applies to startup and trips in service: a condenser that sees cold water slammed onto a hot bundle every shift will loosen tube joints over time, and leaks at the tubesheet after a year of operation are the usual evidence.
Any designation ending in L, M or N, and the letter you choose decides how the rear end is opened for tube cleaning. An M rear is a bonnet that bolts to the tubesheet and must be unpiped to come off; an L rear is a channel with a removable cover, so the tube ends are reached without disturbing the rear piping; an N rear is integral with the tubesheet.
For a condenser with the vapor in the tubes and periodic brushing expected, AEL or AEM with a bolted front cover is the practical choice, giving access at the front and, with L, at the rear. NEN is the lowest cost of all because both ends are welded, and it suits a clean utility or refrigerant condensing duty where nobody plans to open it. Ask for the designation to be written on the quotation so the access you expect is the access you receive.
Yes, and it is the standard arrangement for a tube-side vapor, provided the water is treated well enough to be managed by chemical cleaning alone. Tower water with scale and corrosion control, closed-loop glycol and treated chilled water all qualify.
Raw river, lake or well water does not, because silt settles in the baffle corners and biological film grows on the outside of the tubes where no brush can reach. Hold the shell-side velocity at a level that keeps solids moving, and avoid a design that runs the water below about 3 ft/s in the window areas. A side benefit of water on the shell is that the vapor in the tubes drains to a single outlet channel, which keeps the condensate vent and the noncondensable vent simple. If the only water available is untreated, the unit should be a removable bundle instead.
