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Quotes - Engineering - SalesWhat a plate sizing starts from
A gasketed plate pack is sized from the same heat balance as any exchanger, but the numbers that limit it are different. Send both fluids and their concentrations, both flows, the inlet and outlet temperatures (three of the four plus both flows fixes the fourth), the allowable pressure drop on each side, and the design pressure and temperature. For anything other than water, add viscosity and specific heat at the working temperatures. Then the material inputs: chloride and pH on each side, fouling tendency, and the cleaning method, which together fix the plate grade and gasket compound before the rating starts.
Plate packs run in true counterflow, so they reach approaches of 2–5°F and can run a temperature cross, with the cold outlet above the hot outlet, in a single pass. Their coefficients are high and the plates are thin, which keeps the surface small, and that makes pressure drop, rather than heat transfer alone, the quantity that most often sets the plate pattern and the count. A sizing with no allowable pressure drop is half a sizing.
Finally the mechanical inputs: connection size and type, the space available, and how much duty growth the frame should be able to take.
Sizing assumes 316L plates on clean water and treated loops and titanium where chloride runs above about 1,000 ppm; the plate count changes little between them. Material & Construction explains how the grade is chosen.
Three numbers that drive the plate count
- The closest approach between the two streams
- The allowable pressure drop on each side
- The fouling margin, stated as extra surface
Worked example: cooling process water against a treated loop
This example is illustrative: the clean coefficient is an assumed typical value for water to water, and the real selection is run on rating software against the actual fluids, plate pattern and pressure drop. It shows how the figures fit together, not a promise of performance.
Duty as given
- Hot side: 100 gpm process water, 120°F in, 90°F out
- Cold side: 150 gpm treated loop water, 80°F in
- Allowable drop: 10 psi on each side
- Assumed U: 900 Btu/h-sq ft-°F, clean water to water
- Margin: 15% excess surface
- Plate size: 2.7 sq ft effective area per plate
Arithmetic and result
- Duty: 100 × 500 × 30 = 1,500,000 Btu/h
- Cold outlet: 80 + 1,500,000 / (150 × 500) = 100°F
- LMTD: (20 - 10) / ln 2 = 14.4°F
- Clean area: 1,500,000 / (900 × 14.4) = 116 sq ft
- With margin: 116 × 1.15 = 133 sq ft
- Plates: 133 / 2.7 = 49.3, so 50 thermal plates
Send the duty
both flows and the four temperatures
Set the limits
allowable pressure drop and design pressure
Run the rating
plate pattern, count and pass arrangement
Size the frame
room on the bar for future plates
From duty to plate count
Duty, LMTD and the coefficient
The duty is flow times specific heat times temperature change; for water, 1 gpm raised 1°F carries about 500 Btu/h, which is where the 100 × 500 × 30 in the example comes from. Glycol carries less per gallon, about 4–5% less for 30% propylene glycol and about 10% less at 50%, so the same duty needs more flow. In the example the terminal differences are 20°F at the hot end (120 against 100) and 10°F at the cold end (90 against 80), giving an LMTD of 14.4°F. A single-pass plate pack is close to pure counterflow, so no correction factor is applied; the cold side leaves 10°F above the hot outlet, a temperature cross that a multipass shell-and-tube usually handles only with shells in series.
Pressure drop and the chevron mix
The software calculates U from channel velocity and plate pattern, so U and pressure drop move together. A steep chevron gives a high coefficient and a high drop; a shallow chevron passes the same flow with far less drop and a lower coefficient. Pressure drop rises with roughly the square of channel velocity, so a pack that is too short on drop is fixed by mixing plate angles, choosing a larger plate, or, when one side has far less flow than the other, running that side in two passes.
- Give a real allowable drop per side, typically 5–10 psi on water
- Unequal flows can call for an unequal pass arrangement
- Check the port velocity, which can limit a large plate count
Fouling margin as excess surface
Plate units carry their fouling allowance as excess surface, typically 10–25%, not as a shell-and-tube fouling factor. The reason is arithmetic: a factor of 0.001 h-ft2-°F/Btu added to a clean U of 900 drops it to about 474, which would demand about 90% more plates. Those extra plates lower the channel velocity and wall shear, and lower shear lets deposits settle faster, so over-margined packs often foul sooner than lean ones. Use the low end on treated closed loops and the high end on open tower water or product that films.
Frame size and future plates
A plate pack can grow by adding plates, but only to the length of its carrying bar, the reach of its tie bolts and the capacity of its ports. Ask for a frame with room for 20–30% more plates when the duty may grow, and have the rating state the maximum the frame and connections will take. Added plates raise the duty and lower the pressure drop at the same flow; the pack is then closed to a new tightening dimension for the new plate count.
Plate sizing questions
Plate count is the required area divided by the effective area of one plate, so the same duty can be 25 large plates or 100 small ones. The plate size is chosen from the flow, because the ports must pass it at a sensible velocity, and from the allowable pressure drop. In the example above, 133 sq ft on a 2.7 sq ft plate gives 50 thermal plates; a larger plate would need fewer.
Its coefficient on clean water is typically several times that of a shell-and-tube, it runs in true counterflow, and the stainless wall is thin enough that the conductivity penalty against copper is small. Together those cut the surface sharply for the same duty. The trade is pressure rating, gasket temperature and tolerence for solids, so the comparison only holds where those suit the duty.
Send every case that matters. The plate count is set by whichever case needs the most surface, often the one with the closest approach, and the pressure drop is checked at the highest flow, which may be a different case. A pack sized only for the summer duty can fall short in the winter or free-cooling case, and one sized only for the low-flow case can exceed the pump head in summer.
Plate packs commonly run 2–5°F between the streams. Each degree below that costs surface quickly, because the LMTD shrinks toward zero while the duty stays fixed, so going from a 3°F to a 2°F approach can add a large share of the plates. Choose the approach from what the closer temperature is worth to the plant, then let the rating show what it costs.
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