Plants have flammable gas and salt corrosion; AC must be Ex and resistant.
1. Risk
Flammable and corrosive gas need dual protection.
2. Protection
Ex model + anti-corrosion coat for plant.
3. HCK fit
Ex, C5 anti-corrosion, wide-temp custom.
| Item | Note |
|---|---|
| Risk | flam/corr |
| Rating | Ex+coat |
| Plan | custom |
HCK cabinet AC with Ex and anti-corrosion fits refinery cabinets.
Energy, duty cycle and the ten-year view
Over a ten-year life the energy is typically a larger share of the total cost than the unit itself. That changes which comparison is worth making: not "which is cheaper to buy" but "which will this cabinet consume over a decade", taken from the duty cycle the site will actually see rather than from a catalogue point that assumes continuous running.
- A correctly sized unit cycles. Most of its life is spent at part load, not at its rated point, and that is where a compressor is most efficient.
- Two units at about 60 % each usually beat one at 120 %: they share the duty, they run staggered, and a failure degrades the cooling rather than removing it.
- A fouled condenser is worth roughly a fifth of the capacity, and considerably more on an energy basis, because the compressor fights a warmer surface all year.
- Monitoring that reports duty cycle is usually justified within two seasons, first through the maintenance visits that get avoided.
The counter-argument is first cost, and it is not weak. But the honest comparison is ten-year against ten-year, with the energy figure derived from the site's actual duty rather than from a datasheet assumption that suits the supplier.
Sizing, selection and what the datasheet does not say
The selection that survives a hot summer starts from total heat dissipation rather than from the nameplate rating of the equipment. A drive's nameplate is its output; the heat it puts into the cabinet is a few percent of that, and assuming otherwise is the most common way to oversize a system by a factor of ten.
The working method has four steps. First, list every component that generates heat and take its dissipation from the datasheet rather than from its rating. Second, add the envelope gain of the enclosure itself, which for a sealed cabinet in a hot climate is often the largest single term. Third, apply a safety factor once, to cover the uncertainty in the first two steps. Fourth, round to the nearest sensible unit, and check that the result leaves the unit running part of the time rather than flat out.
| Step | What you are solving | Common error |
|---|---|---|
| Component heat | What the equipment puts into the air inside | Using nameplate power instead of dissipation |
| Envelope gain | Heat from outside through the cabinet walls | Forgetting it on small sealed boxes |
| Safety factor | Uncertainty in the estimate | Applying it twice, or not at all |
| Installed capacity | Rounding to a real catalogue unit | Picking a unit that will run at 100 % duty |
Then ask the three questions that turn a calculation into a specification: what ambient was the quoted capacity measured at, what airflow does the unit need, and how much clearance does the condenser require. Specifications that answer all three tend to be honest about their capacity; specifications that answer none of them are describing a product rather than a performance figure.
Airflow, placement and sealing
A unit that is correctly sized can still fail to hold setpoint because of how the air moves inside the cabinet. The evaporator takes in whatever air is beside it. If the warm air leaving the equipment rises into that intake, the refrigerant has no cold surface to evaporate against and the controller sees a cabinet that will not cool, whatever the rating on the box.
The three symptoms to look for are familiar: the setpoint is reached at the bottom of the cabinet but never near the top, the compressor runs continuously while the panel reads close to ambient, and the unit's own intake temperature is close to the ambient.
- Mount high, where the warm air rises
- Fit a baffle so air passes across the full height of the equipment rather than around it
- Direct the discharge across the load, not down into the floor of the cabinet
- Seal the perimeter between the unit and the cabinet wall; an IP-rated cabinet with an unsealed unit is not an IP-rated cabinet
- Keep the intake grille clean and unobstructed; flow resistance at the intake quietly costs capacity
Measure the temperature spread from the bottom to the top of the cabinet after a full working day. A spread above 8 °C points at the airflow rather than the capacity, and it is a cheaper fault to fix than a larger unit.
Maintenance intervals that fit the site
There is no single correct interval. There is only an interval that is written down, and a filter and condenser that can be reached without dismantling the panel. What a unit in a clean air-conditioned plant room needs and what a unit under a machining centre needs are two different schedules.
| Environment | Filter | Condenser | Drain |
|---|---|---|---|
| Clean indoor plant room | Quarterly | Twice a year | Monthly |
| Workshop with dust or swarf | Monthly | Quarterly | Monthly |
| Outdoor coastal | Quarterly | Every two months | Monthly |
| Desert, sand-laden air | Monthly | Every two months | Monthly |
| Wash-down or high humidity | Monthly | Quarterly | Weekly |
Two habits do most of the work. The first is recording the ambient at each visit, which converts next year's argument about whether the unit has degraded into a settled question. The second is never pressure-washing a dry, dusty coil from the front, which drives the dust into the fins instead of shifting it. Clean dry first, rinse gently, and let it dry before the unit runs again.
Fault diagnosis: symptom, likely cause, what to check
The failure that costs the most is the one that always looks like the compressor. In practice the compressor is rarely the first thing to fail. What fails first is airflow, then the drain, then the sensor, and the compressor is usually simply working against conditions that have quietly become impossible.
| Symptom | Most likely cause | Check first |
|---|---|---|
| Setpoint never reached, compressor runs flat out | Recirculation into its own intake | Intake temperature against ambient |
| Bottom of the cabinet cold, top no warmer than ambient | Air bypassing the equipment | Baffle position and discharge direction |
| Capacity fell without an obvious cause | Fouled condenser | Coil surface temperature and dust build-up |
| Trips in humid weather | Blocked condensate drain | Drain path, and whether it is graded to drain |
| Controller reads a wrong or frozen temperature | Sensor fault | Sensor resistance, then the wiring |
| Ices up inside | Low airflow, or a setpoint below the dew point | Filter condition and setpoint |
Log the duty cycle before any of this. A unit that logged 100 % through the previous summer had the answer to the question already written down, and nobody had read it.
Explosion-protected and hazardous-area enclosures
Where the enclosure sits in a potentially explosive atmosphere, the cooling unit stops being a thermal component and becomes part of the certification. Two consequences follow. First, the unit has to be marked compliant with the standard the project is contracted to, and the documentation has to name the unit being supplied rather than a general family. Second, anything that can become a hot surface is inside a sealed enclosure anyway, so the thermal design and the certification are the same problem examined twice.
In practice the questions worth asking early are: which zone or division the cabinet is in, whether the cooling unit is inside the protected envelope or outside it, what certification the unit itself carries, and how the cabinet is meant to be cooled during maintenance when it is opened. Retrofitting a compliant enclosure after the panel is built is the expensive route, and it is the route most often taken by accident.
Two classes of environment behave very differently in service. A petrochemical site has hazard classification as its dominant constraint and relatively benign ambient conditions. A mine has no hazard classification for the enclosure, but has high ambient on the surface, dust, humidity, and vibration from the equipment around it. Design for the second one as a duty problem and not a certification problem.