Ten guided levels · how cooling works · quick checks
Lab rank Panel Observer· 0 XP
Educational simulation with simplified models — learn the sequence and the feel of the controls here, then follow your site's real procedures on live equipment.
CRAH — chilled water unit
Running
cold airwarm airchilled water supplywarmed water returnbranch bypass (open)
Tap any part — return, filter, coil, fan, supply, sensor, controller, pump, valve or the branch bypass — to see what it does. Air is ducted from overhead: this hall has no raised-floor plenum.
Control panel
Supply fan speed: 85%
CHW valve position: 80%
Branch bypass (BPV): 0%
Supply air setpoint: 20°C
Room heat load: 10%
Live readings
Return air
22.8°C
Supply air
20°C
CHW flow
48gpm
Coil flow
48gpm
Bypass flow
0gpm
Loop delta-T
5.1K
Diff pressure
11.2psi
Cooling demand
7.5kW
Unit capacity
8.7kW
Alarms
All normal
What cooling is actually doing
•Every rack turns electricity into heat, and that heat has to leave the building. Cooling is one continuous path, not a single machine.
•Heat travels: chip → air in the rack → hot aisle return air → the unit coil → chilled water → the chiller evaporator → the chiller condenser → outdoor air through the air-cooled condenser fans.
•Break any link in that chain and the hall warms up. Most alarms are the story of which link is struggling.
CRAC versus CRAH — two different machines
•A CRAC is direct expansion: its own compressor, condenser, expansion valve and evaporator. It makes its own cold, rejects heat through its own air-cooled condenser, and is never connected to the chilled water loop.
•A CRAH has no refrigerant at all. It is a filter, a coil, a supply fan and controls, fed with chilled water from the closed loop.
•So do not look for a chilled water valve, a bypass or loop flow on a CRAC, and do not look for a compressor on a CRAH.
Air-cooled heat rejection
•The chillers are air-cooled: fans move outdoor air across the condenser coil and the loop heat leaves with that air. Nothing evaporates and no water is consumed — there is no cooling tower and no condenser-water loop here.
•Rejection therefore follows the outdoor dry-bulb temperature. A hot afternoon raises condensing pressure, raises the approach, and cuts capacity.
•The approach temperature — condensing temperature minus outdoor air temperature — is the health number for an air-cooled condenser. A fouled coil or a stopped fan shows up there first.
The closed loop and the branch bypass
•The pump circulates the same chilled water from the chillers to every CRAH coil and back. Water leaves the chiller at about 7 °C and returns at about 13 °C; that 5–7 °C is the delta-T, and flow times delta-T is the heat being carried.
•Every branch has a water bypass beside the coil. Its job is minimum loop flow — when coil valves throttle down, the chillers still need enough flow through their evaporators to avoid a low-flow trip — plus freeze protection and coil isolation.
•Left open in normal running, the bypass sends water around the coil. The coil starves, supply air warms, and because the bypassed water returns still cold, the loop delta-T collapses while the chillers work harder than the hall load explains. Warm supply air with a wide-open valve, low coil flow and low delta-T means look at the bypass.
The air path through a unit
•Return air is drawn from the hot aisle into the unit and passes the filter first — filters protect the coil from dust.
•Next is the coil: on a CRAH the heat goes into chilled water, on a CRAC into refrigerant.
•Then the supply fan pushes the cooled air into the overhead supply duct, down into the cold aisle, through the racks, and back to the unit from the hot aisle. There is no raised-floor plenum in this hall.
•Supply air temperature is what you control; return air temperature is what tells you the load.
The controls you actually touch
•CRAH: the chilled water valve modulates flow through the coil, the fan speed sets how much air crosses it, and the bypass stays closed in normal running. Valve and fan work together — opening the valve without airflow cools the unit but not the hall, and running the fan flat out with a closed valve just stirs warm air.
•CRAC: the compressor stages provide the cooling and the fan delivers it. There is no water valve to open, so a DX machine with a warm room means a refrigeration or airflow problem.
•Supply air setpoint is what the unit is trying to hold. The loop setpoint belongs to the chiller plant, and changing it affects every branch at once.
Reading the panel
•Supply air above setpoint means demand is beating capacity: more water, more air, or a fault to find.
•A warm return with a normal supply usually means the hall, not the unit, is the problem — recirculation, an open containment door, or bypass air.
•Low loop delta-T with normal or high flow is a bypass or balance problem; high delta-T with low flow is a restriction or a starvation problem.
•Filter differential pressure rising at a steady fan speed is a loading filter. Plan the change before it starves the coil of air.