DX Unit Load Calculation Worksheet: Cooling Capacity, Airflow & ESP

By SongXin HVAC Engineering Team. SongXin HVAC engineering and technical sales teams support commercial and industrial DX unit selection, air handling equipment review, RFQ checking and export documentation for project buyers.

A good DX unit load calculation starts with three numbers: cooling capacity, supply airflow and external static pressure. For early commercial HVAC selection, calculate the sensible load, add the latent load, convert the result into kW or refrigeration tons, then match the selected DX rooftop unit, ducted DX unit or split DX system against the real duct and filter resistance.

This worksheet is for engineers, contractors and overseas buyers who need a practical pre-RFQ sizing screen before asking for a Direct Expansion Air Conditioners and DX Units quotation. It does not replace a project load calculation by a licensed local engineer. It helps you send cleaner data, avoid undersized equipment and make supplier discussions faster.

Scope boundary: this page is a DX equipment pre-selection worksheet, not a whole-building HVAC design method. Use the broader commercial HVAC system design guide for envelope, zoning, ventilation, system architecture and commissioning decisions; use this worksheet when the project team is ready to turn those loads into DX capacity, airflow and ESP inputs.

DX Unit Load Calculation Worksheet

The worksheet below gives buyers a compact structure for early DX sizing before a full submittal is prepared. Fill in the design values first, then ask the supplier to state the unit selection, fan curve, rating condition and control logic.

Step 1: Define the Design Conditions

DX unit selection should start with actual project design conditions, not the nearest catalog model. The minimum worksheet set is project city, outdoor design temperature, indoor temperature, indoor relative humidity, operating schedule, fresh-air requirement, installation type and power supply.

For a DX rooftop unit, outdoor ambient temperature is especially important because the condenser rejects heat directly to outdoor air. A unit rated at moderate ambient conditions will not deliver the same effective capacity on a hot roof unless the condenser, compressor operating envelope and controls are selected for that condition.

For a DX Split System, refrigerant pipe length and elevation difference must be stated early. Long pipe runs increase pressure drop and oil return risk, so the supplier needs the indoor-to-outdoor distance before final selection.

For Ducted DX Units, the duct layout matters as much as the cooling capacity. If the airflow and ESP are underestimated, the indoor coil will not receive enough air, the supply temperature will drift and compressor protection trips become more likely.

DX Rooftop Units

Roof-mounted packaged cooling.
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Ducted DX Units

Concealed coil with duct distribution.
Explore →

DX Split System

Separated indoor & outdoor sections.
Explore →

Step 2: Estimate Sensible Cooling Load

Sensible cooling load is the heat the DX unit must remove to reduce dry-bulb temperature. It includes building envelope gain, solar gain, lighting, people, motors, equipment, fresh-air sensible load and process heat.

Use this early worksheet structure. The kW values should come from the project load calculation, equipment schedule or consultant’s design file.

Do not add a blanket margin before the loads are separated. A 15% margin on a load that already includes conservative people, lighting and weather assumptions can oversize the DX unit and create short cycling. Oversizing matters because a larger compressor can satisfy temperature quickly while leaving humidity control unfinished.

Step 3: Add Latent Load

Latent load is the moisture removal duty that keeps relative humidity under control. It comes from outdoor air, people, infiltration, wet products, open doors, pools, kitchens, laundries and industrial processes.

For comfort cooling, latent load is often smaller than sensible load. For humid climates, fresh-air systems, kitchens, gyms, medical spaces and process rooms, latent load can become the deciding factor. If a buyer sends only room area and target temperature, the supplier cannot confirm coil selection or condensate drain design.

Step 4: Convert Cooling Capacity

Cooling capacity should be shown in kW and refrigeration tons so engineering and procurement teams can compare international quotations. One refrigeration ton is commonly converted as 12,000 Btu/h, equal to about 3.517 kW, as summarized in the Engineering ToolBox cooling-load conversion reference.

Cooling capacity (TR) = Cooling capacity (kW) / 3.517
Cooling capacity (kW) = Cooling capacity (TR) x 3.517

For early selection, round to the nearest practical model only after rating correction. If the project is in a high ambient region, the selected unit must deliver the required kW at the project outdoor temperature rather than only at a standard catalog rating condition.

Step 5: Calculate Supply Airflow

Supply airflow should be calculated from the sensible load and the planned air temperature difference across the room or coil. A useful SI estimate for early selection is:

Sensible cooling (kW) = 1.2 x airflow (m3/s) x temperature difference (K)

The constant 1.2 is an early design approximation based on air density and specific heat near ordinary comfort conditions. Use psychrometric software or the project consultant’s load calculation for final design.

Rearrange the formula:

Airflow (m3/s) = Sensible cooling (kW) / [1.2 x temperature difference (K)]
Airflow (m3/h) = airflow (m3/s) x 3600

Common early airflow ratios (formula outputs from the sensible heat equation above):

A lower ΔT raises airflow. For the same 60 kW sensible load, 8 K needs about 22,500 m³/h, while 12 K needs about 15,000 m³/h. This is why a DX quote must include both capacity and airflow; kW alone does not tell the engineer whether air distribution will work.

Step 6: Build the ESP Budget

External static pressure is the fan resistance outside the unit casing that the DX unit must overcome at the required airflow. Include supply duct, return duct, filters, dampers, grilles, diffusers, silencers, flexible connectors and any field-installed accessories.

An undersized fan creates two failure modes. First, airflow drops, so the room does not receive the designed sensible cooling. Second, low airflow across the evaporator coil can reduce coil temperature, increase condensate risk and trigger compressor or freeze protection. A DX unit selected without ESP is not a complete selection.

Worked Example

This example shows how to prepare a first-pass RFQ for a commercial ducted DX unit. It uses simplified values for supplier screening; final design should use the project consultant’s load calculation.

Project Inputs

Complete End-to-End Calculation

Formula 1 — total cooling load

Total cooling load = sensible load + latent load
                   = 60 kW + 15 kW
                   = 75 kW

If the loads are early estimates, apply a 10% margin:

Formula 2 — preliminary selection capacity

Preliminary selection capacity = 75 kW x 1.10
                               = 82.5 kW

Formula 3 — kW to refrigeration tons

82.5 kW / 3.517 = 23.5 TR

Airflow Calculation

Airflow = sensible cooling / [1.2 x delta T]
        = 60 / [1.2 x 10]
        = 5.0 m3/s
        = 5.0 x 3600
        = 18,000 m3/h

ESP Calculation

Supply duct and fittings        = 250 Pa
Return duct and fittings        = 150 Pa
Filter final pressure allowance = 150 Pa
Diffusers and grilles           =  50 Pa
Damper allowance                =  50 Pa
Total ESP                       = 650 Pa

When to Choose DX Rooftop, Ducted DX or Split DX

The installation type should follow airflow path, service access, space limits and control requirements. Do not choose the product type only by capacity.

SongXin’s DX product pages cover rooftop, ducted, split, tent and other direct expansion configurations for commercial and industrial projects. Use the worksheet above to make the first supplier conversation specific.

Common Load Calculation Mistakes

DX unit selection fails when the buyer sends area only. Square meters or square feet are insufficient because they do not define solar gain, equipment heat, fresh air, humidity, operating schedule or duct resistance.

Another mistake is mixing sensible and total load. Airflow is driven by sensible load and temperature difference; coil capacity is driven by total load and entering-air condition. When these values are blended into one number, the supplier cannot judge whether the unit will control both temperature and humidity.

A third mistake is ignoring rating standards. AHRI 340/360 is commonly used for commercial and industrial unitary air-conditioning and heat pump equipment, while EN 14511 is commonly used in European rating contexts. Ask suppliers which rating point the capacity comes from.

The last common mistake is leaving filters out of ESP. A clean filter has lower resistance than a dirty filter. If the fan is selected only against clean-filter pressure, airflow can drop as the filter loads. For projects with finer filtration, align filter selection with ISO 16890 classification and include final pressure allowance in the ESP budget.

RFQ Input List for a DX Unit

Send the following data when you ask SongXin for DX unit engineering support:

• Project location and outdoor design temperature.
• Indoor target temperature and relative humidity.
• Sensible load, latent load and total cooling load.
• Fresh-air volume or fresh-air ratio.
• Preferred installation type: rooftop, ducted, split, packaged, tent or remote-site.
• Required airflow in m³/h or CFM.
• External static pressure in Pa, with duct and filter breakdown when available.
• Power supply, control voltage and BMS communication needs.
• Noise limit at occupied area and site boundary.
• Filtration requirement and filter class.
• Refrigerant preference or local refrigerant regulation.
• Installation constraints: roof load, lifting route, plant room space and service clearance.
• Required documents: drawings, wiring diagram, performance data, test report, certificate package and packing details.

Specification Checklist

Use this checklist before approving a DX unit quotation:

Frequently Asked Questions