How to calculate apparatus dew point and bypass factor
How a cooling coil's apparatus dew point and bypass factor predict leaving air condition, and how to construct both on a psychrometric chart from entering conditions.
What this means
A real cooling coil does not bring every molecule of air passing through it into contact with a cold, wet surface. Apparatus dew point is the theoretical saturation condition the coil surface itself sits at, and bypass factor is the fraction of entering air that passes through without contacting the surface at all, arriving at the leaving side unchanged. Leaving air condition is a weighted mix of fully treated air at the apparatus dew point and untreated bypassed air, and calculating it needs both figures together, not either one alone.
Equipment and model context
- Cooling coil selection for ducted air conditioning and heat pump systems
- Worked figures illustrate the method and are not a rating for any product
This explains the concept and the chart construction that connects entering condition, apparatus dew point, and bypass factor. It does not select a coil for a specific application. That requires the coil manufacturer's actual performance data, including its bypass factor at the design airflow, and the entering air condition the system will actually see.
What this covers
- Why leaving air condition is not simply the coil's rated saturation temperature.
- What bypass factor represents physically and how airflow changes it.
- How to construct the apparatus dew point line on a psychrometric chart from entering and leaving conditions.
- Why a coil with a low bypass factor removes more moisture at the same airflow than one with a high bypass factor.
What changes the result
- Assuming leaving air temperature equals the coil's rated apparatus dew point rather than a mix of treated and bypassed air.
- Ignoring that bypass factor changes with airflow, so a coil's performance data at one airflow does not transfer directly to another.
- Selecting a coil by total capacity alone without checking its bypass factor against the sensible and latent split the load actually needs.
- Confusing apparatus dew point, a coil surface property, with the leaving air dew point, which is a mixed condition downstream of the coil.
Why leaving air is not the coil's rated saturation condition
A cooling coil's fins and tubes present a cold, wet surface to the airstream, and air molecules that contact that surface closely approach saturation at the coil's mean surface temperature, the apparatus dew point. Air passing through the gaps between fins without close contact leaves close to unchanged from its entering condition. The stream leaving the coil is therefore a mixture of these two populations, not a uniform condition at either the entering temperature or the apparatus dew point.
This is why a coil's rated apparatus dew point is not the temperature a thermometer reads in the leaving airstream. The measured leaving condition sits between entering condition and apparatus dew point, at a position the bypass factor determines.
What bypass factor represents and what changes it
Bypass factor is the fraction of the temperature and humidity ratio difference between entering air and apparatus dew point that remains unaddressed in the leaving air, expressed as a decimal between zero and one. A bypass factor of 0.15 means the leaving condition sits 15 percent of the way from the apparatus dew point back toward the entering condition, rather than reaching the apparatus dew point itself.
Bypass factor depends on the coil's physical construction, primarily fin spacing and the number of rows in the direction of airflow, more fins and more rows presenting more surface area and lowering bypass factor, and on airflow rate, since higher velocity air spends less time near the coil surface and bypass factor rises correspondingly. A coil's published bypass factor applies at the specific airflow the manufacturer tested it at, not universally across any airflow the coil might see in service.
Constructing the calculation on a psychrometric chart
Plot entering air condition and draw a straight line from it toward the coil's apparatus dew point, which for a coil operating below the dew point of the entering air lies on or near the saturation curve. Leaving air condition falls on that same line, positioned at a fraction from the apparatus dew point back toward entering air equal to the bypass factor: leaving condition equals apparatus dew point plus bypass factor multiplied by the difference between entering condition and apparatus dew point, applied separately to both temperature and humidity ratio.
This construction is what connects the psychrometric chart, discussed on its own, directly to coil selection: the process line a real coil actually draws between entering and leaving air is not an arbitrary diagonal, it is a specific point along the entering-to-apparatus-dew-point line fixed by that coil's bypass factor at the design airflow.
Why this matters for selection, not just analysis
Two coils with identical total capacity ratings can carry different bypass factors, and the one with the lower bypass factor delivers leaving air closer to its apparatus dew point, meaning more thorough sensible cooling and more moisture removal for the same entering condition. Where a load calculation shows a demanding latent requirement, a coil selected partly on bypass factor, not total capacity alone, can meet that requirement more directly than a coil chosen by tonnage without checking it.
Airflow selection interacts with this directly: reducing airflow across a given coil lowers its bypass factor and improves both sensible and latent performance per unit of air moved, which is part of the reasoning behind selecting airflow deliberately for humidity control rather than defaulting to a manufacturer's maximum rated airflow, a point also raised in the sensible and latent load sizing discussion.
For one coil with a bypass factor of 0.15 at its design airflow, this shows entering air, the coil's apparatus dew point, and the resulting leaving air condition on a simplified temperature and humidity ratio plot.
- Entering to apparatus dew point line
- Entering to leaving air, the actual path
- The straight line from entering air to the apparatus dew point represents the condition fully treated air would reach if it contacted the coil surface completely.
- Leaving air actually lands closer to entering air than the apparatus dew point, at a position along that line set by the bypass factor: 15 percent of the distance back toward entering condition in this example.
- A lower bypass factor moves the leaving air point closer to the apparatus dew point itself, delivering more sensible cooling and more moisture removal at the same entering condition and airflow.
- The apparatus dew point itself is a property of the coil and its wetted surface temperature, not a point the leaving air actually reaches unless bypass factor were zero.
| Observation | Explained by | What to check |
|---|---|---|
| Leaving air warmer than the coil's stated apparatus dew point | Bypass factor greater than zero, as expected for any real coil | Confirm the apparatus dew point figure is a coil property, not a leaving-air prediction on its own |
| Reducing airflow improves dehumidification at the same coil | Lower airflow reduces bypass factor | Whether the reduced airflow still meets the sensible capacity the load needs |
| Two coils of equal tonnage perform differently on humidity | Different bypass factors from differing fin spacing or row count | Manufacturer bypass factor data at the actual design airflow for each coil |
Questions people ask about this
Does every coil manufacturer publish bypass factor directly?
Not universally by that name; some publish entering and leaving air conditions at stated test airflows, from which bypass factor and apparatus dew point can be back-calculated using the same chart construction described here. Where the figure is not published directly, deriving it from the entering and leaving data the manufacturer does provide is the practical alternative.
Is a lower bypass factor always better?
A lower bypass factor delivers more thorough treatment per pass, which helps where latent capacity is the binding constraint, but it comes from denser fin spacing or more rows, which raises the coil's air-side pressure drop and therefore the fan energy and available static pressure budget needed to move air through it. The selection is a trade against the rest of the air-side design, not a case of lower being unconditionally better.
Can apparatus dew point be below freezing?
For an air conditioning coil operating on typical comfort cooling entering conditions, apparatus dew point stays well above freezing, but for a coil designed for aggressive dehumidification or an unusual application, calculated apparatus dew point can approach or fall below the freezing point of water, which raises frost risk on the coil surface and needs to be checked against the equipment's actual operating design rather than assumed away.
How does bypass factor relate to filter and coil face velocity?
Face velocity, the airflow divided by the coil's face area, is the direct driver behind bypass factor's dependence on airflow: increasing face velocity for a fixed coil raises bypass factor, while increasing coil face area for the same airflow lowers face velocity and therefore bypass factor. This is one reason a larger coil face area can improve both dehumidification and pressure drop simultaneously, at the cost of the physical space the larger coil occupies.
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This page is awaiting source verification against the documentation in its evidence record: ASHRAE and Air Conditioning Contractors of America technical literature. Its documentation class and intended scope are shown here while that check is pending.
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