Knowledge · Guide

Pressure drop and fan power: the underrated variable

Why the permissible pressure drop dominates the sizing: its link to fan power and operating cost, typical values, and how to choose the limit.

In the enquiry form the permissible pressure drop per side is a required field - not out of formalism, but because it shapes the sizing more strongly than any other single value. Set it too generously and you pay for years in fan electricity; set it too tightly and you get no units at all, or only enormous ones.

Why does pressure drop cost money permanently?

The fan has to overcome every pascal of resistance in every operating hour. The power drawn grows in proportion to volume flow times pressure drop, divided by the efficiency of the fan unit. At 10,000 m³/h, an extra 100 Pa of pressure drop corresponds roughly to 0.4 kW of additional shaft power - over thousands of operating hours a year that adds up to a sum which has to be set against the heat recovered.

How are pressure drop and temperature recovery linked?

They pull in opposite directions. More temperature recovery requires more area or narrower plate gaps - both raise the resistance. The same recovery duty can be solved with a compact unit at a high pressure drop or a larger one at a low pressure drop. The sizing finds, within your pressure drop limit, the sizes with the best recovery; the limit itself is your decision, because only you know your electricity prices, running hours and space.

Which values are usual?

In comfort ventilation, 100 to 200 Pa per side are usually allowed for the heat exchanger; in process plants with powerful fans, more. Two notes from practice:

• Think of both sides separately: the exhaust and outdoor air sides may have different limits - for instance where an existing fan has little reserve on one side.
• Price in fouling: the catalogue value applies to a clean unit. Dusty or greasy extract air raises the resistance in operation; planning without reserve here costs volume flow.

What if nothing fits the budget?

If nothing is achievable within your limit, we tell you the bottleneck. There are then three honest ways out: raise the pressure drop limit (check the fan), lower the recovery requirement, or allow more space - larger edge lengths and parallel modules spread the same volume flow over more cross-section. Which of these is economical we clarify in the quote; the sizing supplies the figures for it.