CRAC Vs CRAH Cooling Coils: What Should OEM Buyers Specify?

When sourcing cooling coils for data center air conditioning equipment, OEM buyers often focus on cooling capacity, coil dimensions, and price. However, one of the most important questions should be answered before requesting a quotation: Is the coil designed for a CRAC unit or a CRAH unit?

Computer Room Air Conditioners (CRACs) and Computer Room Air Handlers (CRAHs) both remove heat from data center air, but they generally use different cooling technologies.

Traditional CRAC units commonly use direct-expansion (DX) refrigeration, in which a refrigerant evaporator coil absorbs heat from the air. CRAH units typically use chilled-water cooling coils connected to a facility cooling water system.

This difference affects the internal coil circuit, operating fluid, temperature conditions, pressure requirements, connections, and selection calculations.

A coil that fits the cabinet dimensions may still be unsuitable if it is designed for the wrong working fluid or operating conditions.

For OEM buyers, accurate coil selection starts with defining the cooling architecture, air-side requirements, fluid-side operating conditions, and mechanical integration constraints.

This guide explains the differences between CRAC and CRAH cooling coils, identifies the technical data suppliers need, and provides a practical RFQ checklist for data center cooling equipment manufacturers and system integrators.

1. What Is the Difference Between CRAC and CRAH Cooling Coils?

1.1 CRAC Cooling Coils: Direct-Expansion Refrigerant Cooling

A Computer Room Air Conditioner commonly uses a direct-expansion refrigeration system.

In a traditional DX configuration, the refrigerant evaporator coil absorbs heat from the return air. The refrigeration circuit then transfers that heat to a condenser and an external heat-rejection system.

The basic process involves:

1. Warm return air passes across the evaporator coil.

2. Refrigerant absorbs heat and evaporates inside the tubes.

3. The air temperature decreases.

4. Refrigerant vapor returns through the refrigeration circuit.

5. Heat is rejected through the system's condenser.

The coil must be designed for the actual refrigerant and its operating conditions.

Important considerations include refrigerant distribution, evaporation temperature, superheat, operating pressure, refrigerant-side pressure drop, and compatibility with the expansion device.

A CRAC evaporator cannot be selected reliably using water-coil performance data alone.

1.2 CRAH Cooling Coils: Chilled-Water Cooling

Air handling units in computer rooms typically use chilled water cooling coils.

These coils receive chilled water from a central cooling system or another suitable cooling water source.

Warm air from the data center flows over the finned surface of the coils while water circulates through the tubes.

The air transfers heat to the water, and the heated water is returned to the facility’s cooling infrastructure.

Unlike traditional direct expansion computer room air conditioners (DX CRACs), standard computer room air handling units (CRAHs) do not require a built-in refrigerant compressor to cool the air.

The selection of the coil is based on factors such as the actual chilled water flow rate, inlet and outlet water temperatures, air conditions, and the allowable pressure drop.

Depending on the specific project requirements, the circulating fluid can be water or an approved water-glycol mixture.

1.3 CRAC vs CRAH Cooling Coils: Key Differences

These comparisons describe conventional configurations. Some precision cooling systems incorporate hybrid refrigeration, economizer coils, or multiple cooling circuits.

OEM buyers should always identify the actual system architecture rather than relying on the CRAC or CRAH product label alone.

1.3.1 Why the Two Coil Types Are Not Automatically Interchangeable

Both types of coils can be constructed using copper tubing and aluminum fins.

However, their visual similarity does not guarantee internal compatibility.

Direct expansion (DX) evaporators require an appropriate refrigerant circuit and two-phase flow distribution, while chilled water coils must provide the required liquid flow and hydraulic characteristics.

1.3.1.1 The Risk of Selecting by Dimensions Alone

Suppose two sets of cooling coils have exactly the same external dimensions and pipe diameters.

One set is designed for a refrigerant evaporator, while the other is designed for a chilled water circulation system.

Their internal circuit connections, pressure requirements, operating characteristics, and test conditions may differ.

Replacing one set with the other without engineering verification could result in poor performance or unsafe operation.

Matching dimensions is necessary for physical replacement, but it is not sufficient for technical interchangeability.

2. What Air-Side Data Should OEM Buyers Specify?

Air-side operating conditions are essential for both CRAC and CRAH coils.

The same coil can deliver different cooling capacities when return-air temperature, airflow, humidity, or air distribution changes.

2.1 Required Sensible and Total Cooling Capacity

Data centers primarily generate sensible heat through IT equipment.

However, cooling equipment selection must still distinguish sensible cooling from total cooling.

Sensible cooling capacity represents the heat removed from the air without moisture removal.

Total cooling capacity includes sensible cooling and any latent cooling associated with moisture condensation.

Buyers should specify both values where applicable.

For example, a requested 60 kW cooling capacity is incomplete if the supplier does not know whether it refers to sensible or total capacity.

The required sensible heat ratio may also need to be established:

Sensible Heat Ratio (SHR) = Sensible Cooling Capacity / Total Cooling Capacity

A high sensible heat ratio is commonly desirable in data center precision cooling, but the actual humidity-control strategy and operating conditions determine the appropriate requirement.

2.2 Entering Air Temperature and Humidity

The estimated dry-bulb temperature of the supply air should be provided to the supplier.

If humidity is a factor, please provide the wet-bulb temperature, relative humidity, dew point, or sufficient psychrometric chart data to determine the supply air conditions.

These values help determine whether the cooling coil will operate in dry or wet conditions.

2.2.1 Dry Coil vs Wet Coil Operation

When moisture has not condensed on the coil surface, the coil is operating in a dry state.

If the coil surface temperature is lower than the air dew point, condensation may occur.

Operating with a wet coil alters the thermal process and may increase air-side resistance.

Proper design depends on whether the unit is intended to provide only sensible cooling, dehumidification, or both.

2.2.1.1 Why Condensation Control Matters in Data Centers

Condensate must be managed to protect surrounding equipment and maintain appropriate environmental conditions.

If wet coil operation is permitted, the original equipment manufacturer (OEM) should specify appropriate measures for condensate collection, drainage, corrosion protection, and maintenance.

For applications where dry operation is planned, the supplier should verify the expected surface temperature conditions rather than assuming that all data center cooling coils will remain dry.

2.3 Design Airflow and Face Velocity

Airflow is a major factor in coil thermal performance.

It is commonly specified in:

• m³/h

• L/s

• CFM

The OEM should identify whether the airflow represents total unit airflow or the actual airflow passing through the coil.

Air bypass around the coil, uneven inlet distribution, and obstructions can change actual performance.

Coil face velocity is calculated from airflow divided by the defined coil face area.

Face Velocity = Airflow / Coil Face Area

Higher face velocity can increase convective heat transfer, but it can also increase air pressure drop and may affect condensate carryover under wet conditions.

The proper value depends on the coil geometry and operating requirements.

2.4 Air-Side Pressure Drop

Air pressure drop is a critical technical parameter because the cooling coil is part of the entire fan system.

Original equipment manufacturers (OEMs) should specify the maximum allowable resistance across the coil at the specified airflow rate.

If the air resistance of the coil is too high, it may increase the fan’s power requirements or prevent the equipment from achieving the target airflow rate.

For a complete technical evaluation, consider:

• Fin spacing

• Number of tube rows

• Tube arrangement

• Airflow uniformity

• Dry or wet operating condition

• Fouling allowance

• Fan performance and available static pressure

The supplier should state whether the quoted pressure drop applies to dry-coil or wet-coil conditions.

3. What Should OEM Buyers Specify for CRAC DX Evaporator Coils?

A CRAC evaporator coil must be selected as part of the complete refrigeration system.

The following parameters should be included in the RFQ.

3.1 Refrigerant Type and Design Pressure

The specific refrigerant must be clearly specified.

Different refrigerants have different pressure-temperature characteristics, thermophysical properties, and safety precautions.

The required mechanical design pressure and applicable test requirements should also be provided to the supplier.

These values must be compatible with the entire refrigeration system.

Without technical verification, never assume that a coil designed for one refrigerant is suitable for another.

3.2 Evaporating Temperature and Refrigerant Conditions

For a DX evaporator, the refrigerant evaporating temperature or relevant saturation condition is an essential selection parameter.

The OEM should also provide the expected refrigerant outlet superheat and other relevant conditions.

Depending on the refrigeration architecture, additional information may include:

• Refrigerant mass flow rate

• Evaporator inlet condition

• Expansion valve or control method

• Refrigerant distribution arrangement

• Acceptable refrigerant-side pressure drop

• Minimum and maximum operating conditions

3.2.1 Why Refrigerant Distribution Affects Coil Performance

A DX evaporator may contain multiple refrigerant circuits connected in parallel.

If refrigerant distribution is uneven, certain circuits may receive different mixtures of liquid and vapor or different mass flow rates.

This reduces heat transfer efficiency and complicates superheat control.

Therefore, as part of the refrigeration system, the coil distributor, circuit layout, and expansion devices must be evaluated.

3.2.1.1 Why More Refrigerant Circuits Are Not Always Better

Increasing the number of parallel circuits can reduce the average refrigerant flow through each circuit.

This may reduce pressure drop under some conditions, but it can also influence refrigerant velocity, oil return, and distribution.

The appropriate number of circuits must be determined using the actual refrigerant and operating range.

A larger number of circuits should not be assumed to improve performance automatically.

3.3 Confirm the Refrigeration System Operating Envelope

The OEM should describe the conditions under which the evaporator must operate.

These may include:

• Maximum design cooling load

• Minimum part-load condition

• Expected air inlet conditions

• Refrigerant evaporating-temperature range

• Compressor capacity-control strategy

• Operating restrictions

• Relevant defrost or frost-prevention requirements

Performance at the nominal design point alone may not be enough for equipment expected to operate continuously over a wide load range.

3.4 Confirm Pressure Testing and Connection Requirements

The refrigerant circuit must be mechanically compatible with the anticipated system pressure and relevant standards.

The buyer shall specify the required pressure and leak test procedures, connection sizes, connection locations, and inspection documentation.

The supplier shall confirm the test requirements for the actual design.

4. What Should OEM Buyers Specify for CRAH Chilled-Water Coils?

CRAH cooling coils require a different set of fluid-side data.

The principal selection questions concern water temperature, flow, hydraulic resistance, fluid chemistry, and operating range.

4.1 Entering and Leaving Water Temperatures

The supplier needs both entering water temperature (EWT) and leaving water temperature (LWT).

For example, an illustrative CRAH cooling coil might receive water at 18°C and return it at 24°C.

The water temperature rise is:

Water Temperature Rise = LWT − EWT = 6 K

This value helps determine the water flow required for a specified sensible cooling duty.

It is only an example, not a recommended standard temperature schedule for all CRAH units.

4.2 Chilled-Water Flow Rate

For a single-phase liquid, heat transfer can be estimated using:

Q = m × Cp × ΔT

Where:

• Q = Heat transfer rate (kW)

• m = Fluid mass flow rate (kg/s)

• Cp = Specific heat capacity (kJ/kg·K)

• ΔT = Liquid temperature change (K)

The flow rate must be sufficient to achieve the required heat transfer under the specified air and water conditions.

4.2.1 What Happens When Water Flow Is Too Low?

Insufficient water flow may reduce available cooling capacity and alter the heat transfer coefficient on the water side.

In addition, this can cause water temperature fluctuations that exceed design expectations.

However, increasing the water flow rate is not always the most effective corrective measure.

A higher flow rate may increase hydraulic resistance and consume more energy for pumping.

4.2.1.1 Why Maximum Flow Is Not the Best Selection Target

A correctly selected coil should satisfy thermal performance and hydraulic limits together.

Excessive water flow can increase pressure drop without delivering a proportionate improvement in cooling capacity.

The appropriate flow should be determined from the actual load, water temperature schedule, coil geometry, and control strategy.

4.3 Allowable Water-Side Pressure Drop

OEMs must specify the allowable pressure drop across both ends of the cooling coil.

This should be coordinated with the chilled water pumping system and the available pressure differential.

For variable-flow CRAH systems, suppliers may also require minimum and maximum design flow conditions.

Quotations should distinguish between the pressure drop of the coil itself and the pressure loss caused by control valves, filters, and the broader piping system.

4.4 Water Quality and Glycol Compatibility

Water quality influences the operating reliability of cooling coils.

The buyer should identify:

• Water treatment requirements

• Glycol type and concentration, if applicable

• Corrosion inhibitors

• Fluid pH and other relevant chemistry

• Filtration requirements

• Fouling expectations

• Material compatibility restrictions

A coil selected for clean water should not automatically be treated as suitable for every water-glycol mixture.

Fluid properties must be included in thermal and hydraulic calculations.

4.5 Water Connections, Drainage, and Venting

Coil connections should be designed for ease of installation and maintenance.

Relevant requirements include connection diameter, location, thread or flange type, flow direction, and venting or drainage devices that facilitate operation.

Original equipment manufacturers (OEMs) should also consider the layout of the supply and return pipes, as well as the ease of operation of nearby control valves.

5. How Do Coil Geometry and Materials Affect CRAC and CRAH Performance?

Although the working fluids differ, both coil types require appropriate air-side construction.

Tube-fin heat exchangers are commonly considered because they offer configurable tube geometry, fin spacing, coil depth, and fluid circuits.

5.1 Copper Tubes and Aluminum Fins

Copper-tube and aluminum-fin construction is a common option for HVAC and refrigeration coils.

The copper tubes carry the working fluid, while the aluminum fins increase the air-side heat-transfer surface.

However, materials should be evaluated for the actual operating conditions.

In corrosive or chemically contaminated environments, additional coatings or alternative materials may be needed.

Aidear offers customizable Tube Fin Heat Exchangers for HVAC, refrigeration, and industrial heat-transfer applications.

5.2 Tube Diameter, Row Count, and Fin Spacing

The coil configuration influences thermal performance, air resistance, and internal pressure drop.

Important variables include:

• Tube diameter and wall thickness

• Tube spacing and arrangement

• Number of tube rows

• Fin pitch and geometry

• Coil face area

• Fluid circuiting

• Header dimensions

A deeper coil with more tube rows may provide additional heat-transfer surface, but it can also increase airflow resistance.

Similarly, closely spaced fins may increase surface density while making cleaning more demanding.

The correct arrangement requires a balance between thermal duty, pressure drop, maintenance access, and dimensional limits.

5.3 Airflow Distribution and Coil Orientation

An OEM should specify the intended airflow direction and installation orientation.

For example, an upflow cabinet, downflow unit, and in-row cooler may have different internal airflow paths and dimensional restrictions.

Uneven airflow distribution can reduce the effective utilization of the coil surface.

5.3.1 Why Nominal Fan Airflow Is Not Always Coil Airflow

The airflow rate specified by the fan manufacturer alone does not guarantee uniform airflow throughout the entire coil.

The cabinet geometry, filters, air intake restrictions, bypass channels, and fan arrangement all affect the actual airflow pattern.

To select the correct model, coil manufacturers need to understand the expected airflow rate through the coil and be aware of the primary sources of airflow non-uniformity.

5.3.1.1 Why OEM Cabinet Drawings Improve Coil Selection

A cabinet drawing with dimensional annotations can reveal installation constraints that are not immediately apparent from the overall dimensions of the coil alone.

These constraints include mounting brackets, connection ports, condensate pans, access panels, and airflow obstructions.

Sharing drawings early in the process can reduce the need for redesigns during the prototype development phase.

5.4 Corrosion Protection and Cleaning Access

Data center cooling coils may operate in controlled indoor environments, but corrosion and fouling risks still require attention.

Potential causes include moisture, incompatible cleaning chemicals, condensate exposure, airborne contamination, and unsuitable water chemistry.

Coating selection should consider the actual exposure and any effect on heat transfer.

OEM buyers should specify how the coil will be inspected and cleaned after installation.

For a related discussion of fin-and-tube coil construction and design trade-offs, read our article on Microchannel vs. Fin-and-Tube Coils for Outdoor Condensers.

6. Worked Example: Preparing a CRAH Cooling Coil RFQ

6.1 Define the Illustrative Air-Side Conditions

Consider a hypothetical CRAH cooling coil with the following design requirements:

The example assumes that the actual air moisture condition and coil surface temperatures permit dry operation.

6.2 Estimate the Sensible Cooling Duty

For an illustrative sensible-only air-cooling process:

Q = ρ × V × Cp × ΔT

Where:

• ρ = Air density (kg/m³)

• V = Volumetric airflow (m³/s)

• Cp = Specific heat of air (kJ/kg·K)

• ΔT = Air temperature reduction (K)

First, convert the airflow:

18,000 m³/h ÷ 3,600 = 5 m³/s

Then calculate:

Q = 1.18 × 5 × 1.005 × (34 − 26)

Q ≈ 47.4 kW

This is an approximate sensible heat calculation for the stated hypothetical conditions.

6.2.1 Estimate the Required Water Flow

Assume entering chilled water at 18°C and leaving water at 24°C.

The water temperature rise is 6 K.

Using an approximate specific heat of water of 4.18 kJ/kg·K:

Water Mass Flow = 47.4 ÷ (4.18 × 6)

Water Mass Flow ≈ 1.89 kg/s

This provides an initial water flow estimate for the assumed duty.

6.2.1.1 Why This Is Not a Final Coil Selection

The calculation does not determine the actual required tube rows, fin spacing, circuit count, dimensions, or pressure drop.

The supplier must also evaluate air-side heat transfer, water-side convection, effective temperature differences, material properties, and coil geometry.

If the coil operates wet, total cooling must be calculated using appropriate moist-air properties rather than the simplified sensible-only equation.

These illustrative values do not represent Aidear product ratings.

6.3 How the RFQ Would Change for a CRAC Coil

If an original equipment manufacturer (OEM) wishes to switch to a DX-type evaporator, the air-side operating conditions remain critical.

However, the purchaser must replace the chilled water parameter table with specific information related to the refrigerant.

This information should include the refrigerant type, evaporation conditions, superheat, design pressure, allowable pressure drop on the refrigerant side, and relevant refrigeration circuit requirements.

This demonstrates why an OEM should not send the same fluid-side specification for both CRAC and CRAH coil inquiries.

7. Complete OEM Cooling Coil RFQ Checklist

The following table can be used when preparing a quotation request.

7.1 Additional OEM Manufacturing Information

To support a complete technical and commercial quotation, buyers should also provide:

• Equipment model or project reference

• Prototype quantity

• Estimated annual order volume

• Installation drawings or CAD dimensions

• Preferred mounting method

• Packaging requirements

• Destination market

• Required technical documentation

• Applicable standards and contractual requirements

• Target delivery schedule

7.2 What the Coil Supplier Should Confirm

The supplier's proposal should identify the conditions used in the equipment selection.

Depending on the scope, this may include:

• Coil construction and dimensions

• Tube and fin material specification

• Design cooling duty

• Air-side pressure drop

• Fluid-side pressure drop

• Connection arrangement

• Relevant temperature and pressure limits

• Required inspection or testing

• Drawing revision and part identification

7.2.1 Prototype and Production Approval

Before volume production, the OEM should establish a clear approval process.

This may include confirming drawings, material specifications, prototype fit, operating performance, and agreed inspection criteria.

7.2.1.1 Why an Approved Drawing Is Essential

An approved drawing should identify the coil revision, dimensions, connection positions, mounting features, and other critical interfaces.

Without controlled documentation, a replacement or repeat-order coil may fit poorly even when its nominal capacity is unchanged.

8. Common Mistakes OEM Buyers Should Avoid

8.1 Requesting Only a Cooling Capacity and Size

A request such as "We need a 50 kW coil with these dimensions" does not provide enough information for reliable thermal selection.

The supplier also needs air-side and fluid-side operating conditions.

8.2 Confusing DX Evaporators With Chilled-Water Coils

CRAC and CRAH coils may look similar but require different circuit and operating specifications.

The actual cooling architecture must be identified.

8.3 Ignoring Air-Side Pressure Drop

A thermally capable coil may still be unsuitable if its airflow resistance exceeds the available fan pressure.

Capacity and airflow resistance must be evaluated together.

8.4 Ignoring Humidity and Condensation

Specifying only dry-bulb temperature can be insufficient when condensation is possible.

The supplier must know the intended wet or dry operating condition.

8.5 Assuming All Copper-Aluminum Coils Have the Same Performance

Coils with the same tube and fin materials can differ substantially in face area, circuiting, fin geometry, row count, and airflow resistance.

Actual performance must be evaluated for the selected configuration.

9. Frequently Asked Questions About CRAC and CRAH Cooling Coils

9.1 Can the Same Cooling Coil Be Used in Both CRAC and CRAH Units?

Not automatically.

Although both may use fin-and-tube construction, a DX evaporator and a chilled-water coil have different fluid-side design requirements.

A common coil platform may be adaptable, but the final circuits and pressure requirements must be engineered for the intended application.

9.2 Is a CRAH Cooling Coil Always More Energy Efficient?

No.

A chilled-water CRAH does not normally have an onboard refrigeration compressor, but the complete system may still rely on chillers, pumps, fans, and other equipment.

System efficiency depends on the overall cooling architecture and operating conditions.

The coil type alone does not determine total facility energy use.

9.3 Can a CRAH Coil Use Glycol Instead of Water?

Yes, if the coil and system are designed for the specified glycol solution.

The selected concentration affects heat-transfer properties, viscosity, pressure drop, and pumping requirements.

9.4 What Is More Important: Coil Capacity or Air Pressure Drop?

Both are essential.

A coil must remove the required heat while operating within the available airflow and fan pressure limits.

Selecting only for maximum capacity can produce an unsuitable design.

9.5 What Does an OEM Need for a Custom CRAC Evaporator Quote?

The buyer should provide refrigerant type, cooling duty, air inlet conditions, airflow, evaporation conditions, superheat requirements, refrigerant pressure limits, coil dimensions, and connection requirements.

9.6 What Does an OEM Need for a Custom CRAH Coil Quote?

The buyer should provide cooling duty, entering and leaving air conditions, airflow, chilled-water inlet and outlet temperatures, water or glycol flow rate, allowable pressure drops, fluid composition, and mechanical requirements.

9.7 Are Tube-Fin Coils Suitable for Data Center Cooling Equipment?

Tube-fin coils are used in various HVAC and precision cooling applications.

Their suitability depends on the required fluid circuit, temperature conditions, airflow, dimensional constraints, and mechanical specifications.

The coil must be selected for the actual equipment rather than assumed suitable for every CRAC or CRAH design.

10. Conclusion: Specify the Coil for the Cooling Architecture

CRAC and CRAH cooling coils perform a similar air-cooling function, but they generally use different internal working fluids and design principles.

A conventional CRAC evaporator requires refrigerant-specific circuiting and operating data.

A CRAH cooling coil requires a chilled-water or glycol temperature schedule, fluid flow rate, and hydraulic design information.

Both types also depend on accurate air-side specifications, including airflow, entering air conditions, required cooling duty, and allowable pressure drop.

For OEM buyers, the most effective selection process is to provide complete operating conditions before discussing final dimensions and pricing.

The right cooling coil is not simply the one that fits the cabinet. It is the one that meets the required thermal duty, hydraulic limits, operating conditions, and mechanical integration requirements.

10.1 Discuss Custom Tube-Fin Cooling Coils With Aidear

Changzhou Aidear Refrigeration Technology Co., Ltd. supplies customized tube-fin heat exchanger solutions for HVAC, refrigeration, industrial cooling, and related applications.

For OEM projects involving precision cooling equipment, the appropriate coil configuration should be evaluated using the actual working fluid, cooling capacity, airflow, operating temperatures, pressure limits, and installation requirements.

Explore Aidear Tube Fin Heat Exchangers and our Custom Copper Tube and Fin Heat Exchanger product options to learn more about customizable coil structures.

Specific CRAC or CRAH operating compatibility, pressure ratings, and performance must be confirmed for each proposed design.

10.2 Request a Custom CRAC or CRAH Cooling Coil Quote

Are you developing a CRAC unit, CRAH system, or customized data center air cooling product?

Send Aidear your required cooling capacity, entering air conditions, design airflow, fluid or refrigerant specifications, allowable pressure drops, coil dimensions, and equipment drawings.

Our team can review the supplied requirements and discuss suitable tube-fin heat exchanger configurations, subject to technical evaluation and manufacturing feasibility.

Request a Custom Quote — Share Your CRAC or CRAH Cooling Coil Requirements With Aidear.