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Quotes - Engineering - SalesHow a brazed plate selection is put together
Every brazed selection starts with the duty, Q = mass flow × specific heat × temperature change. For water the shortcut is 500 Btu/h per gpm per °F; glycol carries less, about 4–5% less heat per gallon for 30% propylene glycol and about 10% less at 50%, and its viscosity lowers the film coefficient as well. The temperature driving force is the LMTD of a true counterflow layout, which is why brazed units hold close approaches of a few degrees. The overall coefficient on clean water-to-water duty is high, often 600–1,200 Btu/h-ft2-°F, because the chevron channels stay turbulent at low flow. Area is then Q / (U × LMTD), plus a fouling margin stated as excess surface, typically 10–25%, rather than a TEMA fouling factor.
Area becomes a plate count. Effective area per plate times the number of plates less two gives the surface, and the flow divides among the channels on each side. Channel type is the second lever: a high-angle, hard channel gives more transfer and more pressure drop, a low-angle, soft channel less of both, and mixed or asymmetric channels suit unequal flows. The selection settles where both circuits meet their duty within the pressure drop each side can spend. More allowable pressure drop means fewer plates; too little allowable drop means a large, slow pack that fouls in the 1–3 mm channel.
Two-phase duty is sized by zone. An evaporator needs the refrigerant, its saturation temperature, the superheat and the distribution at the inlet; a condenser needs the desuperheating, condensing and subcooling zones, each with its own temperature difference. Design pressure follows the refrigerant: subcritical CO2 evaporators run roughly 130–410 psig, and transcritical gas coolers need units rated about 2,030 psig. The two-phase page covers those circuits.
Sizing runs on 316L plates; the copper, nickel or stainless braze follows the fluids and barely changes the thermal result, but water chloride decides whether brazed fits. See Material & Construction for the grade comparison.
What a brazed sizing needs from you
Both circuits
- Each fluid and its concentration, glycol type and percentage
- Flow and inlet and outlet temperature on each side, or the duty and three of the four temperatures
- Allowable pressure drop on each side, from the pump curve if possible
- Whether either side is evaporating or condensing, and the refrigerant
Limits and chemistry
- Design pressure and temperature on each side, with the code required
- Water analysis: chloride, pH, hardness, ammonia, sulfide
- Particles, fibers or scale that could reach a narrow channel, and the strainer
- Copper limits that call for nickel or stainless braze
The installation
- Connection size and type: threaded, solder or weld stubs
- Mounting position and space for the pack and its insulation
- Dual-circuit or double-wall requirements from the system design
- Growth: a sealed pack cannot take added plates later
Worked example: water to water at 750,000 Btu/h
Illustrative only: U, plate area and pressure drop come from rating software for a specific plate, and the quotation, not this arithmetic, states the performance of the unit.
| Feature | Typical build |
| Hot side | 50 gpm process water cooled from 110 to 80°F |
| Duty | 50 × 500 × 30 = 750,000 Btu/h |
| Cold side | 60 gpm closed-loop water at 70°F; 750,000 / (60 × 500) = 25°F rise, out at 95°F |
| LMTD | End differences 15°F and 10°F; (15 - 10) / ln(15/10) = about 12.3°F |
| Clean area | U assumed 800 Btu/h-ft2-°F; 750,000 / (800 × 12.3) = about 76 ft2 |
| Fouling margin | 15% excess surface; about 87 ft2 |
| Plate count | About 1.0 ft2 effective per plate: 87 plus 2 end plates, 89 plates (44 channels per side) |
| Channel flow | About 1.1 gpm per channel hot side and 1.4 gpm cold side; pressure drop then checked against each allowance |
Brazed sizing questions
Because extra plates slow the flow in every channel. Lower velocity means lower turbulence, a lower film coefficient and faster fouling in a 1–3 mm gap, and on a controlled loop it makes the temperature hunt at part load. A 10–25% surface margin covers normal fouling. If the duty itself is uncertain, size for the highest realistic case rather then padding the plate count.
Two ways. A 30% propylene glycol carries about 4–5% less heat per gallon than water, so more flow is needed for the same duty, and its higher viscosity, especially cold, lowers the film coefficient and raises pressure drop. The pack grows by more than the heat-capacity difference suggests. At 30% the freeze point is about +9°F; send the concentration actually in the loop, not the design figure.
Whatever the pump can spare, stated for each side. Many water circuits are selected in the 3–10 psi range per side; less than that forces a large, slow pack, while more buys a smaller one at the cost of pump energy. On a refrigerant side the allowance is set as a saturation temperature penalty instead, because pressure drop there lowers the effective temperature difference.
No. The pack is a sealed block, so the plate count is fixed once it is brazed. If the load is expected to grow, either size the brazed unit for the future duty now and accept part-load operation, or choose a gasketed plate unit, which can take added plates in the same frame when the load arrives. We can quote both for comparison.
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Why an evaporator or condenser pack is built differently from a liquid unit: inlet distribution, asymmetric channels, port direction and the freeze line.
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