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Quotes - Engineering - SalesThermal Sizing
Rating Against Your Actual Duty
A thermal rating balances the heat transfer requirement against the pressure drop available, on both sides, at the fouling condition you expect.
The design point is the easy part. A selection is judged on the range around that point: how far the unit turns down, how much the cooling water swings between seasons, and the transient it sees at start-up or on an upset.
Pressure drop is the usual omission. A unit selected on thermal duty alone can meet the temperature and be impossible to pump through at the flow the system delivers. It is the most common cause of a rejected selection.
Fouling allowance. Over-generous allowance produces excess surface and low velocity, which encourages the deposition it was meant to cover. Real experience beats a default value.
Typical Specification
- Thermal inputs: Both fluids, flows, inlet and outlet temperatures
- Hydraulic inputs: Allowable pressure drop each side
- Mechanical inputs: Operating and design pressure and temperature
- Fouling: From experience where available
- Turndown: Range as well as design point
- Seasonal: Cooling water temperature variation
- Materials: Selected against stream and cleaning chemistry
- Output: Plate count, pattern, frame and connection selection
- Expansion: Frame space specified where growth is expected
- Documentation: Rating summary supplied with the quotation
Common FAQs
Because it usually decides the selection. A unit that meets the thermal duty and exceeds the available pressure drop is unusable, and that is the most common reason a selection has to be redone.
As much as your experience supports. Excessive allowance produces an oversized unit running at low velocity when clean, which encourages the very fouling it was meant to cover.
For both. A unit sized only on peak can be poorly controlled at the load it actually runs at most of the time, which is a real operating problem rather than a theoretical one.
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