What Is a CRAC Unit?
A computer room air conditioner (a CRAC unit) is a precision cooling system that maintains stable temperature, humidity, and airflow in a data center or server room. It works as a self-contained air conditioner. A compressor circulates refrigerant through an evaporator coil, the coil absorbs heat from warm air returning off the server racks, and a condenser rejects that heat outside the building.
The comparison that matters most in data center design is between a CRAC unit and a CRAH unit. A CRAC makes its own cooling with a compressor and refrigerant. A CRAH unit, or computer room air handler, has no compressor at all.
Both sit in the white space and move air through the same racks. What differs is where the cooling comes from, and that changes how the equipment is sequenced, staged, and controlled.
How a CRAC Unit Works
A CRAC unit operates a direct-expansion (DX) refrigeration cycle. Four things happen in sequence:
- Warm return air from the hot aisle passes across the evaporator coil. Liquid refrigerant inside the coil boils off, absorbing heat from the air.
- The compressor raises the pressure and temperature of that refrigerant vapor.
- The condenser rejects the heat outside the conditioned space.
- The refrigerant expands back to a liquid and returns to the coil.
Beyond the refrigeration circuit, a CRAC unit includes supply fans, usually electronically commutated (EC) fans on current equipment, air filtration, and often a humidifier and reheat stage so the unit can hold a dew point rather than only a dry bulb temperature. That humidity control is the reason precision cooling equipment exists as a category separate from comfort HVAC.
Heat Rejection Explained
Heat rejection is where CRAC unit HVAC design branches into three configurations:
- Air-cooled: Refrigerant piping runs to a condenser mounted outdoors. Simplest to install, most common in smaller rooms.
- Water-cooled: A condenser water loop carries heat from the unit to a cooling tower. Common where a facility already operates a central condenser water system.
- Glycol-cooled: A water-and-glycol mix circulates to an outdoor dry cooler, which avoids freeze risk in cold climates and removes the water-treatment load of an open tower.
The second and third options are why cooling towers and CRAC units so often appear on the same mechanical schedule, and why the control strategy has to account for equipment outside the computer room. M-Tech builds cooling tower control panels for the same central plants that reject CRAC and CRAH heat.
How a CRAH Unit Differs
A CRAH unit is a coil, a fan array, filtration, and a control valve. Chilled water arrives from a central plant, typically at supply temperatures in the mid-40s to mid-50s Fahrenheit, depending on design. Warm return air crosses the coil, heat transfers to the water, and the warmed water returns to the chillers.
Capacity is set by two things: how far the modulating valve opens and how fast the fans run.
Removing the compressor eliminates the largest electrical load and the most maintenance-intensive component. It also moves the mechanical cooling work to a central plant, where large chillers operate more efficiently than a fleet of small distributed compressors. The tradeoff is infrastructure. A CRAH needs chillers, pumps, piping, and usually cooling towers before the first unit runs.
CRAC vs. CRAH Unit Comparison
| CRAC unit | CRAH unit | |
|---|---|---|
| Cooling medium | Refrigerant (direct expansion) | Chilled water |
| Compressor | Yes, in or adjacent to the unit | No |
| Heat rejection | Air-cooled condenser, cooling tower, or dry cooler | Central chiller plant, usually with cooling towers |
| Capacity control | Compressor staging, digital scroll or variable speed compressors, fan speed | Modulating chilled water valve, fan speed |
| Typical facility load | Commonly applied below roughly 200 kW of IT load | Commonly applied at 200 kW and above |
| Up-front cost | Lower. No central plant required | Higher. Chillers, pumps, and piping come first |
| Operating efficiency | Lower. Distributed compressors carry the mechanical load | Higher. No compressor in the unit, central plant runs at better efficiency |
| Maintenance | Refrigerant charge, compressor oil, coils, filters | Coils, filters, valve and actuator |
| Typical delta T across the unit | Legacy DX units around 18°F | Modern chilled water coils reach 10°F to 25°F |
| Best fit | Edge sites, telecom rooms, small server rooms, retrofits into buildings with no chilled water | Colocation, hyperscale, enterprise halls, anywhere a central plant already exists |
Where CRAC and CRAH Units Sit in the Cooling Loop
Neither unit works alone. Each one is the terminal device at the end of a heat rejection chain, and the chain determines what the controls have to coordinate.
DX CRAC with air-cooled condenser: server exhaust → CRAC evaporator coil → compressor → refrigerant line → outdoor air-cooled condenser → ambient air.
Water or glycol-cooled CRAC: server exhaust → CRAC evaporator coil → compressor → condenser water or glycol loop → pump → cooling tower or dry cooler → ambient air.
CRAH with central chiller plant: server exhaust → CRAH chilled water coil → chilled water return → chiller evaporator → chiller condenser → condenser water pump → cooling tower → ambient air.
Every arrow in those sequences is a point where something has to be commanded, measured, or interlocked. That is the work a control panel does.
Temperature and Humidity Targets
ASHRAE publishes two sets of limits, and the difference between them drives the control strategy.
The recommended envelope is where the equipment should normally sit. The allowable envelope is how far conditions can drift before hardware is at risk. The 2021 edition of the Equipment Thermal Guidelines for Data Processing Environments sets the recommended range at 64.4°F to 80.6°F (18°C to 27°C), measured at the equipment inlet, with a relative humidity of up to 60%.
That recommended range is identical across equipment classes A1 through A4. Only the allowable range changes. Class A1 hardware tolerates 59°F to 89.6°F, while class A4 tolerates 41°F to 113°F, so a room of A4 equipment has more headroom before a cooling problem becomes a hardware problem.
Hitting those numbers is as much an airflow problem as a capacity problem. The servers themselves raise the air temperature 20°F to 35°F on the way through.
How to Tell Where Air is Going
Two temperature comparisons show whether air is going where it should, and both should stay under 5°F:
- Unit supply air against equipment intake air: A wide gap means hot exhaust is recirculating into the cold aisle and warming the supply before the servers get it.
- Equipment exhaust air against unit return air: When return air reads cooler than exhaust, conditioned air is bypassing the racks and going straight back to the unit without cooling anything.
Choosing Between a CRAC and a CRAH Unit
Two questions settle it. How much IT load does the room carry, and does chilled water already exist in the building?
- Below roughly 100 kW, a CRAH rarely justifies itself because the chillers, pumps, piping, and usually cooling towers must be paid for before the first unit cools anything.
- Above roughly 200 kW, that reverses, and a fleet of small distributed compressors from CRACs becomes an expensive way to make cold air.
Between those figures, the site decides. Where chilled water already serves other building loads, a CRAH is straightforward to justify. Where the room is a retrofit into a building with no central plant, a CRAC is usually the practical answer. Sites expecting to expand tend toward the CRAH path, since adding units to an existing plant costs less than adding compressors.
How Control Panels Run CRAC and CRAH Units
A precision cooling unit is only as good as the logic commanding it. A control panel for CRAC and CRAH equipment handles:
- Capacity Modulation: Compressor staging and unloading on a CRAC, chilled water valve position on a CRAH, with fan speed managed through EC fan signals or a VFD.
- Sensing: Supply and return air temperature, humidity, and differential pressure across the floor or containment. Supply air control tracks what the servers actually receive, which is why it has largely replaced return air control as the primary strategy.
- Humidity Management: Humidifier and dehumidification sequencing, coordinated across units so one is not adding moisture while its neighbor removes it.
- Redundancy Sequencing: N+1 and 2N standby logic, lead and lag rotation to balance runtime hours, and automatic failover on unit alarm.
- Unit-to-Unit Coordination: Networked units share sensor data and a common setpoint so adjacent equipment does not fight, a failure mode that wastes energy and shortens compressor life without triggering a single alarm.
- Monitoring and Integration: Condensate and leak detection, filter differential alarms, and reporting to the building management system or DCIM platform over BACnet or Modbus.
Panels for this environment are built to UL 508A, carry a marked short circuit current rating as required by NEC Article 409.110, and use enclosures rated for the space they occupy.
Build the Controls Around Your Equipment
M-Tech Control designs and manufactures custom control panels for the cooling equipment that keeps data centers within their thermal envelope, from single-unit retrofits to multi-door panels for a full central plant.
Every panel is built to UL 508A and engineered to the sequence of operations the facility actually runs. Request a quote to talk through your specifications today.
Frequently Asked Questions About CRAC and CRAH Units
What does a control panel do for a CRAC or CRAH unit?
It executes the sequence of operations. That includes staging compressors or modulating a chilled water valve, controlling fan speed through EC fans or a VFD, reading temperature, humidity, and differential pressure sensors, managing humidification, sequencing N+1 redundancy and lead-lag rotation, detecting condensate leaks, and reporting to a building management system over BACnet or Modbus. It also coordinates networked units so adjacent equipment shares a setpoint instead of working against each other.
Does a CRAH unit need a cooling tower?
Usually, yes. A CRAH unit depends on a chiller plant, and water-cooled chillers reject their heat through a condenser water loop to cooling towers. Air-cooled chillers are the exception and reject heat directly to ambient air.
Can CRAC and CRAH units run in the same data center?
Yes, and it is common in facilities that have grown in phases. Mixed rooms put more weight on the control strategy, because equipment with different capacity control methods has to hold a single supply air target without hunting.