Psychrometry describes the thermodynamic properties of moist air. A psychrometric chart turns those properties into a visual design map, helping a refrigeration coil buyer connect entering and leaving air, humidity, dew point, sensible and latent load, airflow, condensate, and frost risk.
The phrase psychrometry and psychrometric charts can sound academic, but the practical question is usually straightforward: what must the air-side coil do, under which conditions, and how will the supplier prove it? A chart helps the project team see the air-state change. It does not select a coil by itself.
For a custom evaporator, cooling coil, air cooler, or related heat exchanger, the chart should sit beside the drawing, load calculation, fluid data, pressure limits, and test plan. That combination gives procurement a comparable RFQ and gives engineering a clear list of assumptions to review.
What psychrometry means for a refrigeration project
Psychrometry is the study of moist-air properties and processes. The air is a mixture of dry air and water vapor, so its state cannot be described by dry-bulb temperature alone. The ASHRAE Handbook chapter on psychrometrics explains that the subject uses thermodynamic and transport properties to analyze moist-air conditions and processes.
In a refrigeration coil project, the most useful properties are:
- Dry-bulb temperature, the ordinary air temperature measured by a suitable sensor.
- Humidity ratio, the mass of water vapor per unit mass of dry air.
- Relative humidity, the ratio of the current vapor condition to saturation at the same temperature.
- Dew-point temperature, the temperature at which condensation begins for the stated air pressure and moisture content.
- Wet-bulb temperature, a property affected by evaporation and airflow around a wetted sensor.
- Enthalpy, a convenient way to represent the total heat content of the moist-air stream on the chosen basis.
The chart places several of these properties on one visual field. The selected chart is not universal, however. Pressure and altitude affect the relationships, and a chart made for standard atmospheric pressure may not represent a high-altitude application accurately enough for a final selection.
How to read a psychrometric chart
A psychrometric chart normally uses dry-bulb temperature as one reference axis, with other property lines crossing it. The saturation curve marks air that is fully saturated at the chart pressure. Relative-humidity curves sit below that boundary. Depending on the chart, humidity ratio, wet-bulb temperature, enthalpy, and specific volume are shown with their own line families.
The chart is most useful when the project plots a state point rather than when it treats the whole graphic as a generic illustration. A state point should have a source, unit system, pressure basis, measurement location, and design status. For example, 鈥渞eturn air at the coil inlet鈥?is more useful than 鈥渞oom air,鈥?because filters, fans, outdoor-air mixing, heaters, and upstream coils may change the actual condition at the coil face.
When the entering and leaving states are connected, the line provides a visual description of the air-side process. A mostly horizontal change may represent sensible cooling on a particular chart. A move down and left can include moisture removal. A process that reaches a surface condition below freezing needs a separate frost and defrost review, even if the air-state line appears simple.
What the chart can show about coil duty
The first task is to define the two air states. Record the entering dry-bulb temperature and either relative humidity or humidity ratio. Record the required leaving condition in the same way. Add airflow, barometric pressure or altitude, and the location of each measurement.
For a cooling coil, the total air-side duty can be represented in a form such as Q = m虈da(hin - hout), where the property basis and units must be stated. The sensible portion relates mainly to dry-bulb change. The latent portion relates to the change in moisture content and the condensate that leaves the air stream. A supplier may use a different calculation convention, so the RFQ should ask for the property source, dry-air basis, unit system, and assumptions.
The chart supports four common coil discussions:
| Air-side process | What the buyer should check |
|---|---|
| Sensible cooling | Dry-bulb change, airflow, face velocity, pressure drop, and surface temperature. |
| Cooling with dehumidification | Dew point, humidity-ratio change, condensate rate, drain pan, slope, and corrosion exposure. |
| Low-temperature or frosting service | Coil surface condition, fin spacing, frost allowance, defrost method, drainage, and fan control. |
| Mixed or staged air | Outdoor-air fraction, return-air condition, mixing location, bypass, and the state entering each coil. |
The chart does not provide the complete coil geometry. It cannot decide tube diameter, circuiting, fin material, header size, casing, connection layout, refrigerant distribution, or mechanical support without additional design inputs. It also does not prove capacity. Those decisions require the working fluid, operating pressure, flow conditions, construction, and an agreed rating or test method.
The data needed before plotting a coil process
An RFQ or design review should distinguish measured values, calculated values, customer targets, and supplier proposals. The distinction matters when a replacement coil has to fit an existing unit or when a new coil will be tested under a different condition from the one shown on the initial chart.
At minimum, prepare the following information:
- Identify the application, such as a cold room, freezer, process cooler, air handler, chiller, condenser, or dedicated dehumidification stage.
- State entering and leaving air conditions, airflow, pressure or altitude, outdoor-air fraction, and the measurement locations.
- Define the heat load, operating schedule, design ambient, target approach, and any normal or worst-case operating point.
- State the refrigerant or secondary fluid, concentration where relevant, evaporating or condensing condition, flow rate, and design pressure.
- Provide the equipment envelope, drawing revision, face dimensions, row depth, connections, mounting points, service clearances, drain route, and airflow direction.
- Identify the materials, coatings, cleaning chemistry, salt exposure, humidity, food or hygiene requirements, and dissimilar-metal interfaces.
- Describe frost, defrost, condensate, fan or pump control, filter condition, and the performance value that must be checked.
If the chart is built from a software tool, retain the input file or calculation record. A screenshot without the property method, pressure basis, and units is difficult to audit later. The NIST reference on algorithms for psychrometric calculations is a useful reminder that property calculations have a defined method and source.
Using psychrometric charts in an RFQ
The chart should make the RFQ more precise, not make it longer for its own sake. Put the plotted state points and their data table near the front of the request. Then connect each state change to the coil requirement and acceptance evidence.
| RFQ input | Why the supplier needs it |
|---|---|
| Entering and leaving air state | Establishes sensible and latent duty and makes the plotted process reviewable. |
| Airflow and pressure basis | Sets mass flow, face velocity, air-side pressure drop, and the property basis. |
| Refrigerant or secondary fluid | Controls circuiting, pressure, temperature, materials, controls, and safety review. |
| Coil surface and frost condition | Determines whether condensate, frost, defrost, fin spacing, and drain design must be considered. |
| Mechanical envelope | Protects fit, connections, supports, access, fan interface, and replacement compatibility. |
| Testing and documentation | Defines pressure or leak checks, dimensional records, thermal evidence, certificates, and deviations. |
Ask every supplier to return a marked assumption register. If one quote uses a different altitude, entering humidity, airflow, bypass factor, or refrigerant condition, the prices are not yet comparable. The supplier should also identify whether the result is a preliminary selection, a guaranteed rating, a prototype proposal, or a final production design.
For general forced-circulation air-cooling and air-heating coils, AHRI 410 can be a useful reference for rating discussions. Its published scope includes important exclusions, including frosting conditions, microchannel coils, bare-tube coils, several refrigerant system arrangements, and non-round tube coils. Do not treat an AHRI reference as automatic coverage for every industrial refrigeration application. State the actual project method and exclusions in the purchase specification.
Common mistakes in psychrometric chart work
Treating relative humidity as moisture quantity
Relative humidity changes with temperature. Air can have the same moisture content at two different temperatures and show different relative-humidity values. For load and condensate reasoning, humidity ratio or another explicitly defined moisture basis is often more useful than relative humidity alone.
Ignoring pressure and altitude
Charts are built for a stated pressure. ASHRAE publishes charts for different pressure conditions, and the correct chart depends on the application. If a project is at elevation or the air pressure differs materially from standard conditions, note that before using a plotted state for a final design.
Plotting only one nominal point
A coil may see seasonal ambient changes, part load, filter fouling, frost growth, defrost recovery, or a changed outdoor-air fraction. Plot the normal point and the condition that controls capacity, pressure drop, safety, or moisture management.
Using a chart as a capacity guarantee
A chart describes an air-side state change. It does not verify refrigerant distribution, fluid-side pressure drop, fin efficiency, leakage, mechanical strength, or manufacturing quality. These items need their own calculations, inspection points, and test records.
Mixing property bases or units
Enthalpy and humidity ratio may be expressed per unit mass of dry air or on another stated basis. Keep IP and SI units separate, and ask suppliers to show the conversion or property source when a quotation uses a different convention.
How Domi can support a coil review
Domi can review a psychrometric chart together with the drawing, sample photographs, operating conditions, fluid data, material requirements, connections, testing scope, packaging, and quantity ladder. The useful output is a short assumption register that separates confirmed inputs, open questions, and proposed supplier options.
For a starting point, see Domi’s industrial refrigeration coils page and the commercial refrigeration coil sizing guide. For a replacement or OEM project, the custom refrigerator coil RFQ checklist helps organize drawings, dimensions, connections, and commercial requirements. A custom project can continue through custom coil fabrication after the technical basis is clear.
Send the current drawing revision, the entering and leaving state data, airflow, fluid and pressure information, material requirements, quantity, destination, and requested documents. Ask the supplier to state what is included, what remains open, and which change would require a new approval. That process makes the chart useful to engineering and procurement at the same time.
Frequently asked questions
What is psychrometry used for in refrigeration?
Psychrometry is used to analyze moist-air properties and processes. In refrigeration, it helps connect air temperature, humidity, dew point, enthalpy, airflow, sensible load, latent load, condensate, and frost risk.
What does a psychrometric chart show?
A chart can show relationships among dry-bulb temperature, humidity ratio, relative humidity, wet-bulb temperature, dew point, enthalpy, and sometimes specific volume for a stated pressure range.
Does a psychrometric chart select a coil by itself?
No. It supports the air-side design basis. Coil selection also needs refrigerant or secondary-fluid conditions, capacity, pressure drop, geometry, materials, connections, controls, frost or condensate requirements, and a validation method.
How does dew point affect an evaporator coil?
When the coil surface is below the entering-air dew point, water can condense on the coil. That affects condensate handling, drain design, corrosion, hygiene, fin spacing, insulation, and service requirements.
Can a psychrometric chart show frost?
It can show the moisture condition that contributes to frost, but frost growth, defrost timing, drainage, fan control, and capacity recovery require a separate coil and system analysis.
Why can two suppliers quote different coil capacity?
They may use different airflow, pressure, humidity, property data, bypass assumptions, refrigerant conditions, surface temperatures, or rating points. Align those inputs before comparing capacity or price.
What should be included with a psychrometric chart in an RFQ?
Include the state-point data, units, pressure basis, airflow, application, fluid conditions, drawing revision, target duty, operating range, frost or condensate expectation, test requirements, and the assumptions behind the plotted process.
Can Domi review a chart for a custom coil project?
Domi can review the chart as part of a broader coil inquiry. Send the plotted states together with the operating data, drawing or sample, materials, connections, quantity, packaging, and requested documentation so the next engineering question is clear.
Next step for buyers
If the project is ready for quotation, send the chart and its source data with the current drawing revision and operating envelope. If the chart is still preliminary, label it as such and ask the supplier to identify the missing inputs before a selection is treated as final. A clear distinction between a design target and a verified rating protects both procurement and engineering.
For a Domi review, use the custom coil fabrication inquiry or contact the team. Include the application, entering and leaving air state, airflow, refrigerant or secondary fluid, pressure, dimensions, connection layout, quantity, destination, and required test records.
Related articles
| Article | Use it for |
|---|---|
| Commercial refrigeration coil sizing | Capacity, face velocity, and pressure-drop inputs. |
| How to size a custom HVAC coil | A broader coil sizing and RFQ framework. |
| Cooling coils | HVAC cooling and dehumidification coil selection. |
| Industrial refrigeration evaporator selection | Evaporator duty, airflow, frost, and defrost questions. |
| Forced-air cooler selection | Cold-storage and produce pre-cooling applications. |
| Custom refrigerator coil RFQ checklist | Drawing, sample, material, and quotation preparation. |
