Sizing Cabinet AC Cooling Capacity: Heat-Budget Method

Heat-Budget Formula

Total capacity Q_tot = Q_int + Q_cond + Q_inf, then multiply by a 1.2–1.5 safety factor. Q_int sums heat from VFDs, supplies and PLCs; Q_cond is wall conduction plus solar gain; Q_inf is door/seam infiltration.

Worked Example

Item	Value
Internal heat Qint	520W
Conduction+solar Qcond	180W
Infiltration Qinf	60W
Subtotal	760W
×1.25 safety	950W

Sizing Notes

• Use datasheet heat-loss, not rated power, for components.
• For direct sun or ambient >45℃, use factor 1.4+.
• Pick HCK-1000 (1000W) to cover 950W with margin.

查看 HCK 机柜空调产品 或 联系昊诚工控 获取选型方案。

4. A worked example, metric units

A plant integrator needs cooling for a drive cabinet in a foundry. The cabinet is 2.2 x 0.8 x 1.2 m, the ambient is 38 °C, the target internal temperature is 26 °C, and the equipment inside dissipates 2,960 W.

Step 1 — surface area

A = (2.2 x 0.8 + 2.2 x 1.2 + 0.8 x 1.2) x 2 = 39.5 m²

Step 2 — envelope heat gain

Q_env = 5.5 x A x ΔT = 5.5 x 39.5 x (38 - 26) = 2,607 W

Notice what this means: before a single drive is energised, the box itself is asking for 2.6 kW. On a small PLC panel that figure is routinely larger than the components.

Step 3 — component heat load

Q_i = 2,960 W, taken from drive and power-supply dissipation rather than from nameplate current.

Step 4 — apply the safety factor

Q_total = 1.1 x (2,960 + 2,607) = 6,124 W

Two 3,200 W units give 6,400 W of installed capacity — about 5 % spare, which is the right ballpark for a cabinet that will not change.

Step	Calculation	Result
Surface area	(2.2x0.8 + 2.2x1.2 + 0.8x1.2) x 2	39.5 m²
Envelope gain	5.5 x 39.5 x 12	2,607 W
Component load	given	2,960 W
With safety factor	1.1 x (2,960 + 2,607)	6,124 W
Selected	two 3,200 W units	6,400 W

5. The same example converted to BTU / hour

Purchasing departments and US panel shops usually work in BTU/hr. The conversion is simple: BTU/hr = watts x 3.412.

• Envelope gain: 2,607 W x 3.412 = 8,894 BTU/hr
• Component load: 2,960 W x 3.412 = 10,100 BTU/hr
• With safety factor: 6,124 W x 3.412 = 20,897 BTU/hr

So the same cabinet is a ~21,000 BTU/hr application. If a supplier quotes you 300,000 BTU/hr for that panel, they sized it from the main isolator's current rating.

6. Why undersizing fails quietly

An undersized unit rarely fails in a way anyone notices on day one. It runs at 100 % duty for the whole hot season. And compressor capacity is at its worst exactly at 100 % duty — the refrigerant mass flow is fixed by the motor, so the heat it can lift falls away as ambient climbs. The result is a cabinet that gets hotter precisely in the worst week of the year, and a drive that trips on its own thermal cutoff long before the cooling unit would ever look wrong.

The expensive version of the same mistake is over-sizing from a nameplate rating. You pay for capacity that never gets used, and you get short cycling instead of spare cycling.

7. Site adjustments you must apply before selecting

Condition	Effect on the calculation
Direct solar radiation on the cabinet	Add 10–30 % to the load
Ambient above 40 °C	Derate the selected unit by 10–20 %
Altitude above 1,000 m	Derate about 3 % per additional 100 m
Poor internal circulation	Add margin, and map the temperature distribution
Cabinet sealed against the building	Reduce the envelope term and cap the gain

8. Five mistakes that account for most bad selections

1. Sizing from full-load current instead of dissipation. Typically overestimates by 5–20x.
2. Forgetting the envelope term entirely. On a large cabinet this is the dominant load.
3. Applying one safety factor to the total and never to the site conditions.
4. Ignoring duty cycle — assuming a unit rated for intermittent duty will run all summer.
5. Placing the unit so its intake faces the cabinet's hot side, then blaming the capacity.

9. Two smaller units instead of one large one

Two half-size units are usually the better purchase, for three concrete reasons. They share the load so each runs part-time and lasts longer. They survive a single failure with degraded but non-zero cooling. And they can be staged — one unit for spring and autumn, both for a heat wave — which saves energy for most of the year.

10. Verifying the selection after commissioning

• Log the unit's duty cycle for one full week in the hottest month.
• Map internal temperature at the top, at the bottom and at the hottest component.
• If the spread is above 10 °C, the airflow is wrong, not the capacity.
• If duty is 100 % and the setpoint is being met, size up for next summer.

11. Reading the numbers back into the cabinet

A sizing calculation is only as good as the load it was given. The calculation returns a number in watts; the cabinet has to be built around that number. Three places where the two drift apart:

• The dissipation figures come from a datasheet at 25 °C. Drives dissipate more at high ambient because the semiconductor junction and the cooling fan both work harder.
• The envelope term assumes steady conditions. A cabinet opened for ten minutes an hour does not see the envelope at all; it sees ambient air inside the sealed volume.
• The safety factor gets applied twice, or not at all. One pass at 1.1 covers uncertainty in the load estimate; a second pass on top of that oversizes the unit and costs energy for a decade.

Input	Typical source of error	Effect on the result
Component dissipation	Datasheet at 25 °C rather than at site ambient	Understates load in hot sites
Surface area	Inside concealed panels not counted	Understates envelope gain
Ambient	Recorded indoors when the cabinet sits outside	Understates everything
Safety factor	Applied twice, or omitted	Overstates, or understates capacity
Duty assumptions	Treated as continuous when the process is intermittent	Oversizes the unit

12. Six sizing mistakes and how each one shows up

1. Sizing from the drive's nameplate instead of its dissipated power. A 22 kW drive might dissipate 900 W. Selecting 22 kW of cooling is a 24-fold overestimate.
2. Ignoring the envelope. In a small sealed cabinet at 40 °C ambient the box itself can be the largest heat source, larger than everything inside.
3. Forgetting the safety factor. A load list is an estimate; the factor is what covers the error in the estimate.
4. Selecting against room temperature, not the component's limit. A VFD may tolerate 40 °C and a battery 35 °C. Cooler is not automatically safer if it costs a permanently running compressor.
5. Assuming the datasheet ambient. The quoted capacity applies at a stated ambient. A unit selected at 25 °C and installed outdoors in the Gulf will not hold its rating.
6. Not checking the airflow path. A perfectly sized unit that recirculates its own heat performs like one half its size.

Every one of these is caught by the same three questions: what is the load, what is the ambient, and where does the air go.