The Billion-Rand Question Nobody’s Asking
The physics of condenser airflow that transport refrigeration installers refuse to calculate
Formula 1 teams spend over R4 billion annually developing their cars. They employ hundreds of aerodynamicists, run thousands of hours of computational fluid dynamics simulations, and test relentlessly in wind tunnels costing R50 million each. Every gram of weight, every square centimetre of surface area, every watt of energy is scrutinised, calculated, and optimised.
And every single one of them encloses their coolers.
Not some of them. Not the ones with bigger budgets. All of them. Mercedes, Ferrari, Red Bull, McLaren—teams that would gain any competitive advantage possible—universally house their radiators inside carefully ducted bodywork with precisely engineered inlet and outlet paths.
NASCAR does the same. IndyCar does the same. The World Endurance Championship prototypes do the same. Rally cars competing in 40°C desert conditions do the same.
Meanwhile, walk into any transport refrigeration installer in South Africa and suggest enclosing a condenser behind a fairing, and you’ll hear the same objections repeated like religious doctrine:
“The coil needs to be in open air.”
“Ram air from driving helps the cooling.”
“Enclosures trap heat.”
Someone is wrong here. Either billion-rand racing programmes with unlimited engineering talent have collectively made a fundamental error, or the transport refrigeration industry is perpetuating myths that cost operators money while delivering inferior performance.
This article will demonstrate, with calculations the industry refuses to perform, exactly who is wrong and why. We’ll examine the physics of condenser airflow, quantify the “ram air” that installers worship, and explore design philosophies that could transform transport refrigeration performance—if anyone bothered to engineer them properly.
We’ve covered the aerodynamic costs that refrigerated truck operators pay every year and the design revolution that could save R150,000 per vehicle. Now it’s time to address the component that installers treat as sacred and untouchable: the condenser itself.
The Three Myths of Open-Air Condensers
Before we can discuss better designs, we need to demolish the folklore that prevents progress. The transport refrigeration industry has repeated these claims so often that they’ve become accepted wisdom—despite being demonstrably false.
Myth #1: “The Coil Needs Maximum Airflow Exposure”
This sounds intuitively correct. More air touching the coil means more cooling, right?
Wrong. What matters is not air around the coil, but air through the coil.
When you mount a condenser in “open air” without ducting, approaching air does what all fluids do when encountering an obstacle: it takes the path of least resistance. That path is around the condenser, not through it.
The condenser coil creates resistance to airflow—that’s inherent in its design. Hundreds of fins, multiple tube passes, tight spacing. Air approaching an open-mounted condenser sees this resistance and mostly diverts around the edges. Only a fraction actually passes through the heat exchange surfaces.
This fraction is called capture efficiency, and for open-mounted condensers, it’s shockingly low: typically 15-30% of the theoretical airflow actually passes through the coil. The rest flows uselessly around the perimeter, contributing nothing to heat rejection.
A ducted condenser, by contrast, forces approaching air through the coil because there’s no alternative path. Capture efficiency rises to 70-90%. Same ambient airflow, same condenser, but three times more air actually doing useful work.
Racing teams understood this decades ago. That’s why their radiators sit inside ducted enclosures with sealed perimeters. Not because they have excess cooling capacity to waste, but because ducting dramatically improves the efficiency of the capacity they have.
Myth #2: “Ram Air From Driving Helps Cooling”
This is the myth that sounds most scientific. Vehicle motion creates dynamic pressure. Dynamic pressure pushes air. Therefore, faster driving means better cooling.
The physics is correct. The magnitude is not.
Dynamic pressure follows a simple formula:
q = 0.5 × ρ × V²
Where:
- q = dynamic pressure (Pa)
- ρ = air density (kg/m³)
- V = velocity (m/s)
At 80 km/h (22.2 m/s) in Johannesburg (air density 0.95 kg/m³ at 1,750m altitude):
q = 0.5 × 0.95 × (22.2)²
q = 0.5 × 0.95 × 493
q = 234 Pa
This is the theoretical maximum—the pressure you’d measure at a stagnation point where air comes to a complete stop against a surface. For an open-mounted condenser, you don’t get this. You get the effective dynamic pressure after accounting for capture efficiency.
With 25% capture efficiency on an open mount:
q_effective = 234 Pa × 0.25 = 59 Pa
Now compare this to what the condenser fan provides. A typical transport refrigeration condenser fan generates 50-100 Pa of static pressure. At highway speed—the best-case scenario for ram air—the effective ram air pressure roughly equals what the fan provides anyway.
But here’s where the myth completely falls apart: courier and delivery operations don’t spend much time at highway speed.
We analysed a typical multi-stop courier route with 15 deliveries over a 6-hour period. Here’s the time-weighted breakdown:
| Activity | Duration | % of Route | Speed | Effective Ram Air |
|---|---|---|---|---|
| Loading at depot | 30 min | 8% | 0 km/h | 0 Pa |
| Urban driving | 120 min | 33% | 25 km/h | 5 Pa |
| Delivery stops | 90 min | 25% | 0 km/h | 0 Pa |
| Suburban arterial | 60 min | 17% | 50 km/h | 29 Pa |
| Highway sections | 30 min | 8% | 80 km/h | 59 Pa |
| Traffic/waiting | 30 min | 8% | 0 km/h | 0 Pa |
Time-weighted average effective ram air pressure: 11 Pa
Compare this to the fan’s constant 50-100 Pa contribution. Across a typical courier duty cycle, the fan provides 82-90% of the total driving pressure. Ram air contributes the remainder—when it’s available at all.
But here’s the cruel irony: ram air is available when you need it least and absent when you need it most.
When is your thermal load highest? When you’re stationary with the doors open, making a delivery on a 35°C summer afternoon. Ram air at that moment: zero.
When is ram air at maximum? Highway cruising at 120 km/h with doors closed, cargo stabilised, minimal thermal load. You have maximum “free” airflow precisely when you don’t need it.
The ram air myth persists because it feels true. Motion equals cooling equals good. But the numbers reveal it’s a minor contributor at best and completely absent during peak thermal demand.
Myth #3: “Enclosures Trap Heat”
This objection sounds like basic thermodynamics. Hot condenser inside a box. Heat accumulates. System fails.
The error is treating the enclosure as a static container rather than a flow-through system.
Let’s do the energy balance that installers never calculate.
Heat sources inside an enclosure:
- Solar radiation on enclosure surface: ~500 W/m² peak, affecting maybe 0.5 m² = 250W
- Conduction from engine bay (if poorly sealed): 100-200W
- Heat rejected by condenser: 3,000-8,000W (this is the entire point of the system)
Heat removal capacity:
Even at stationary with fan only, a properly designed system moves approximately 0.9 kg/s of air through the condenser. With even a modest 5°C temperature rise across the heat exchanger:
Q_removal = ṁ × Cp × ΔT
Q_removal = 0.9 kg/s × 1.005 kJ/kg·K × 5K
Q_removal = 4.5 kW
The ratio:
- Heat input (worst case): 250W + 200W = 450W
- Heat removal capacity: 4,500W
- Removal exceeds input by 10:1
Heat only “traps” if:
- The fan fails completely (zero airflow)
- Recirculation approaches 100% (fundamental design failure)
- Complete blockage of inlet or outlet
None of these occur with proper ducting design. The enclosure doesn’t trap heat—it channels airflow to ensure heat is removed efficiently.
The actual temperature rise inside a properly designed enclosure with a 5kW cooling load:
ΔT = Q / (ṁ × Cp)
ΔT = 5 kW / (0.9 kg/s × 1.005 kJ/kg·K)
ΔT = 5.5°C
Enclosure interior: 35°C ambient + 5.5°C rise = 40.5°C
This is completely acceptable and does not impair condenser function. The condenser is designed to reject heat with condensing temperatures of 50-60°C; a 40°C enclosure temperature provides ample thermal gradient for effective operation.
What Actually Cools Your Condenser
With the myths addressed, let’s establish what actually determines condenser performance.
Heat rejection from a condenser follows the fundamental equation:
Q = ṁ × Cp × ΔT
Where:
- Q = heat rejection rate (kW)
- ṁ = mass flow rate of air through condenser (kg/s)
- Cp = specific heat capacity of air (1.005 kJ/kg·K)
- ΔT = temperature rise of air across condenser (K)
The temperature rise is constrained by the condensing temperature and ambient temperature. If your refrigerant condenses at 55°C and ambient is 35°C, you have a maximum ΔT of 20K available (in practice, you won’t achieve this across the entire coil, but it sets the upper limit).
This leaves mass flow rate as the primary variable you can influence through design.
Mass flow rate depends on:
ṁ = ρ × A_coil × V_face × η_capture
Where:
- ρ = air density (kg/m³)
- A_coil = condenser face area (m²)
- V_face = air velocity approaching condenser (m/s)
- η_capture = capture efficiency (fraction of approaching air that passes through coil)
You can’t change air density (it’s determined by altitude and temperature). You can specify condenser face area (bigger is better, within packaging constraints). You have limited control over approach velocity (it’s determined by vehicle speed and fan performance).
But capture efficiency? That’s entirely determined by installation design. And it’s where open-mount installations fail catastrophically.
Open mount: η_capture = 0.15-0.30 Ducted installation: η_capture = 0.70-0.90
Same condenser. Same fan. Same vehicle speed. But a ducted installation delivers three times more air through the coil than an open mount.
This is why racing teams enclose their coolers. Not because they have excess capacity, but because ducting transforms the capacity they have into actual heat rejection.
For detailed calculations and worked examples, see our Technical Formulas Reference.
The Recirculation Problem Nobody Mentions
There’s another factor that makes open-mount condensers worse than their rated specifications: recirculation.
When hot exhaust air from the condenser can re-enter the inlet, it raises the effective ambient temperature the condenser sees. The condenser isn’t rejecting heat to 35°C ambient; it’s rejecting heat to a mixture of 35°C ambient air and 40°C exhaust air.
Recirculation impact formula:
T_effective = T_ambient + (T_exhaust - T_ambient) × R
Where R is the recirculation fraction (0 to 1).
Open-mount recirculation factors:
| Condition | Recirculation | Why |
|---|---|---|
| Stationary, calm | 10-20% | Hot air rises, partially re-enters |
| Stationary, crosswind | 20-40% | Wind pushes exhaust toward inlet |
| Driving, following wind | 5-15% | Exhaust carried forward |
| Driving, headwind | 0-5% | Best case for open mount |
Consider a hot summer day: 35°C ambient, condenser raises air temperature by 5.5°C to 40.5°C at exhaust. With 30% recirculation (moderate crosswind at stationary):
T_effective = 35 + (40.5 - 35) × 0.30
T_effective = 35 + 1.65
T_effective = 36.65°C
Available ΔT drops from 20K to 18.35K—an 8% capacity reduction just from recirculation.
In severe crosswind conditions with 40% recirculation, the capacity loss reaches 11%.
Ducted systems virtually eliminate recirculation by physically separating inlet and exhaust paths. With proper design, recirculation drops below 2% regardless of wind conditions. The condenser sees true ambient temperature, delivering rated capacity consistently.
This is why open-mount condenser performance varies wildly with conditions while ducted systems deliver predictable, consistent results. The installer who says “it works fine” tested it on a calm day with the vehicle pointing into whatever breeze existed. They didn’t test it in a crosswind. They didn’t test it stationary in a car park with buildings disrupting airflow.
Design Philosophy #1: Exhaust Up and Over
Having established that ducted installations outperform open mounts, let’s examine how to implement them. There are several fundamental approaches, each with distinct characteristics.
The conventional wisdom—when installers do consider ducting at all—is to exhaust upward. Hot air rises naturally, so help it along by directing exhaust over the roof of the vehicle.
The Upward Exhaust Approach
Airflow path:
- Inlet on fairing front face or cab roof trailing edge
- Air passes through condenser
- Fan pushes air upward
- Exhaust exits above loadbox roof level
Advantages
Buoyancy-assisted at stationary: When the vehicle isn’t moving, natural convection helps. Hot air at 40°C is less dense than ambient at 35°C. The buoyancy pressure assisting upward flow:
P_buoyancy = g × h × (ρ_ambient - ρ_hot)
For a 0.3m exhaust path height with 5°C temperature differential:
ρ_ambient (35°C) ≈ 1.15 kg/m³
ρ_hot (40°C) ≈ 1.13 kg/m³
Δρ = 0.02 kg/m³
P_buoyancy = 9.81 × 0.3 × 0.02
P_buoyancy ≈ 0.06 Pa
This is tiny—essentially negligible—but it’s assisting rather than opposing the fan. Every bit helps at stationary where thermal loads are highest.
Simple ducting geometry: Upward exhaust aligns with the natural fairing profile. Air enters the front, passes through the condenser, exits the top. No complex turning or redirection required.
Industry familiarity: When installers do implement ducted systems, upward exhaust is what they know. Reduced resistance to adoption.
No underbody exposure: Exhaust path stays above the chassis, avoiding road spray, debris, and the hostile underbody environment.
Disadvantages
Exhausting into neutral or positive pressure zone: At highway speed, the flow over the cab roof creates a boundary layer. Pressure in this zone is approximately neutral (same as ambient) or slightly positive. You’re not gaining any pressure assistance from vehicle motion.
Lower pressure differential:
At 80 km/h:
- Inlet (fairing face): +234 Pa (stagnation pressure)
- Outlet (above roof): ~0 Pa (boundary layer)
- Available differential: ~234 Pa
This is the dynamic pressure at the inlet only. At lower speeds, it drops proportionally.
Potential recirculation over cab: In certain wind conditions, exhaust air can be carried forward over the cab and re-enter the inlet zone. This is particularly problematic with tailwinds where vehicle motion doesn’t clear exhaust effectively.
Adds vehicle height: Depending on implementation, upward exhaust ducting may increase the overall vehicle profile, affecting clearance for multi-storey car parks and covered loading docks.
When Upward Exhaust Makes Sense
- Long-haul applications with minimal stops (sustained highway speed, lower emphasis on stationary performance)
- Vehicles where underbody routing is impractical due to chassis configuration
- Retrofit applications where minimising modification complexity is prioritised
- Operations primarily in open areas with consistent wind patterns
Design Philosophy #2: Exhaust Down to Underbody
Here’s where engineering analysis leads us somewhere counterintuitive. What if, instead of exhausting upward where hot air “wants” to go, we exhausted downward into the underbody airstream?
The Physics Favouring Downward Exhaust
At highway speed, the underbody experiences significant negative pressure due to the Venturi effect. Air accelerates as it’s squeezed between the vehicle floor and the road surface. Faster air means lower pressure.
Typical underbody pressures at 80 km/h:
| Zone | Pressure | Why |
|---|---|---|
| Centreline underbody | -80 to -120 Pa | Basic Venturi effect |
| Between wheels (sides) | -120 to -180 Pa | Stronger channel effect |
| Wheel wake areas | -50 to -100 Pa | Turbulent, less predictable |
Now calculate the pressure differential for downward exhaust:
Inlet (fairing face): +234 Pa
Outlet (underbody): -100 Pa (conservative estimate)
Available differential: 334 Pa
Compare to upward exhaust: 234 Pa
Downward exhaust provides 43% more pressure differential at highway speed.
This additional pressure differential translates directly to improved airflow without increasing fan power. Or, viewed differently, you can achieve the same airflow with a smaller, quieter, more efficient fan.
The Cab-to-Box Gap Connection
We’ve written extensively about the aerodynamic disaster of the cab-to-loadbox gap. Air hitting the front of the loadbox creates enormous drag. But some of that air doesn’t just create pressure—it has to go somewhere.
The airflow pattern in the cab-to-box gap naturally tends downward. High pressure at the loadbox face, lower pressure below the chassis. Air follows this gradient whether you want it to or not.
Downward condenser exhaust aligns with this natural flow pattern. Instead of fighting the aerodynamics of the gap, you’re using them. Your exhaust air joins the flow already heading under the truck, getting swept away from the vehicle with zero recirculation risk.
What About Buoyancy?
Yes, downward exhaust fights buoyancy at stationary. Hot air wants to rise; you’re pushing it down.
Let’s quantify this opposition:
Buoyancy pressure (5°C temperature rise, 0.3m path):
P_buoyancy ≈ 0.06 Pa (calculated earlier)
But this now OPPOSES the fan rather than assists.
Wait—0.06 Pa? The fan provides 50-100 Pa. Buoyancy opposition is less than 0.1% of fan pressure.
Even if we’re more conservative and estimate buoyancy at 10-15 Pa (accounting for longer exhaust paths and higher temperatures), the fan overcomes this trivially.
At stationary, the fan does all the work regardless of exhaust direction. Buoyancy assistance from upward exhaust is negligible. Buoyancy opposition from downward exhaust is equally negligible. The fan dominates.
But at highway speed? Downward exhaust captures an additional 100+ Pa of pressure differential that upward exhaust wastes.
Disadvantages of Downward Exhaust
Road spray and debris exposure: Exhaust paths leading to the underbody must be protected from water, mud, and road debris ingestion. This requires:
- Downward-facing exit orientation (not upward-facing scoops)
- Louvers or baffles to prevent backflow
- Drain provisions for any water that enters
- Corrosion-resistant materials
More complex ducting: The airflow path requires turning from horizontal (or vertical through condenser) to downward to underbody. This adds components and installation complexity.
Less intuitive: Installers trained on “hot air rises” will resist downward exhaust as illogical. Overcoming this requires education and demonstration.
Clearance constraints: Exhaust ducting must not reduce ground clearance or interfere with chassis components, suspension, or drivetrain elements.
When Downward Exhaust Makes Sense
- Courier and delivery operations with frequent stops (maximises benefit from improved pressure differential during driving phases)
- Vehicles already implementing cab-to-box fairings (natural integration opportunity)
- Operations prioritising aerodynamic efficiency and fuel economy
- New-build vehicles where ducting can be incorporated from design stage
Design Philosophy #3: Dual Angled Exhaust
Taking the downward exhaust concept further, what if we optimised not just the direction but the specific destination of the exhaust air?
The Side-Underbody Advantage
Recall the pressure distribution under a moving vehicle:
| Zone | Pressure at 80 km/h |
|---|---|
| Centreline underbody | -80 to -120 Pa |
| Side underbody (between wheels) | -120 to -180 Pa |
| Wheel wake | -50 to -100 Pa |
The lowest pressure zones are at the sides, where the channel effect between wheels and body is strongest. By directing exhaust specifically to these zones, we capture even more pressure differential.
Dual Fan Configuration
Instead of a single central fan exhausting straight down, consider two fans angled outward:
Front View:
CONDENSER
┌─────────┴─────────┐
│ │
┌────▼────┐ ┌────▼────┐
│ FAN 1 │ │ FAN 2 │
│ ╲ │ │ ╱ │
│ ╲25° │ │ 25°╱ │
└─────╲───┘ └───╱─────┘
╲ ╱
╲ ╱
▼ ▼
Left side Right side
underbody underbody
Pressure differential with 25° outward angle:
Inlet (fairing face): +234 Pa
Outlet (side underbody): -150 Pa
Total differential: 384 Pa
Compare to straight-down: 334 Pa Compare to upward exhaust: 234 Pa
The dual angled configuration provides 64% more pressure differential than upward exhaust.
Additional Benefits of Dual Fan Configuration
- Redundancy: Single fan failure means 100% capacity loss. With dual fans, one failure means 50% capacity—degraded but operational. You can complete the route and address the problem later rather than facing a roadside breakdown.
- Better condenser coverage: A single 400mm fan creates non-uniform airflow across a wide condenser. Centre gets good flow; edges are starved. Two 280mm fans, each covering half the condenser, deliver more uniform flow across the entire face. Uniform flow means uniform heat transfer means better overall performance.
- Packaging advantages: Two smaller fans require less vertical clearance than one large fan. This is particularly relevant when mounting behind angled fairings where space is constrained.
- Noise reduction: Same total airflow at lower individual fan speeds. Noise scales approximately with the fifth power of tip speed, so modest speed reductions yield significant noise benefits.
- Eliminated cross-stream interference: With outward-angled exhaust, the two exhaust streams diverge immediately. There’s no possibility of them colliding and creating backpressure. Each stream is captured by the side underbody suction independently.
Zero Recirculation
Perhaps the most significant advantage: dual angled exhaust makes recirculation physically impossible.
- Exhaust exits at the bottom of the fairing
- Immediately directed outward at 25° from vertical
- Captured by side underbody airstream
- Carried laterally and rearward, away from vehicle
There is no path—under any wind condition, at any vehicle speed—for exhaust air to return to the inlet at the fairing front face. Recirculation drops to effectively zero.
This means the condenser sees true ambient temperature in all conditions. Rated capacity becomes actual delivered capacity.
Fairing Integration: The Missed Opportunity
So far we’ve discussed exhaust strategies as if the condenser installation were independent of the vehicle aerodynamics. But the greatest gains come from integrating condenser ducting with aerodynamic fairing design.
We’ve explained the fairing effect that nobody calculated—how a large condenser mounted as a fairing can actually reduce drag compared to a small roof-mounted unit on a flat loadbox wall. Now let’s take this further.
Current Industry Approach: Siloed Thinking
Fairing design: “Make a smooth shape to reduce the cab-to-box discontinuity.”
Condenser installation: “Bolt the unit somewhere accessible. Roof mount is easiest.”
These are treated as completely separate problems, solved by different suppliers with no coordination. The result is predictable:
- Fairing provides aerodynamic benefit
- Condenser installation compromises that benefit
- Neither achieves optimal performance
Integrated Approach: The Fairing IS The Condenser Housing
What if the fairing wasn’t just an aerodynamic shell but served as the condenser enclosure?
- Fairing front face: Becomes the inlet for condenser airflow. High-pressure stagnation zone feeds directly into the system.
- Fairing internal volume: Houses the condenser, fan(s), and ducting. The wedge-shaped space between curved fairing exterior and flat loadbox wall provides natural plenum geometry.
- Fairing structure: Supports condenser weight, provides weather protection, integrates mounting points.
- Exhaust routing: Incorporated into fairing lower surfaces, directing flow to optimal underbody zones.
One component serving multiple functions. Weight is used efficiently. Aerodynamics and thermal management optimise together rather than compromising each other.
The Horizontal Coil Advantage
Integrated fairing design enables a condenser orientation that’s nearly impossible with conventional mounting: horizontal.
Consider the space available in a typical cab-to-box fairing:
The wedge-shaped volume has:
- Width: 1.6-1.9m (between chassis rails)
- Depth: 0.4-0.6m (fairing depth front-to-back)
- Height: 0.3-0.5m (varies with fairing profile)
Compare condenser orientations:
| Orientation | Usable Dimensions | Coil Area |
|---|---|---|
| Vertical (against loadbox wall) | 1.6m × 0.35m | 0.56 m² |
| Angled 25° from vertical | 1.6m × 0.40m | 0.64 m² |
| Horizontal | 1.6m × 0.55m | 0.88 m² |
Horizontal orientation delivers 40-60% more condenser area within the same fairing envelope.
More area means lower face velocity for the same airflow. Lower face velocity means:
- Lower pressure drop across the coil
- Better heat transfer (more contact time)
- More capacity headroom for altitude, ambient extremes, door openings
The 90° Turn: Manageable Engineering
Horizontal condenser mounting requires air to flow vertically through the coil (down, in the case of downward exhaust). But the inlet is on the fairing face, delivering air horizontally. Somewhere, the airflow must turn 90°.
Installers will object that this turn “wastes pressure” and “reduces efficiency.” Let’s calculate how much.
Pressure loss in a 90° turn:
ΔP_turn = K × 0.5 × ρ × V²
Where K is the loss coefficient depending on turn geometry:
| Turn Design | K Factor | ΔP at 5 m/s |
|---|---|---|
| Sharp 90° (square corners) | 1.2 | 14 Pa |
| Radiused corners (r/D = 1.0) | 0.25 | 3 Pa |
| Turning vanes | 0.20 | 2.4 Pa |
| Expansion + turn + contraction | 0.15 | 1.8 Pa |
With proper design—radiused corners, adequate plenum volume, perhaps simple turning vanes—the 90° turn costs 2-3 Pa.
Available pressure differential with downward exhaust: 334 Pa (or 384 Pa with angled exhaust).
The turn consumes less than 1% of available pressure. This is not a meaningful loss; it’s engineering noise.
The horizontal coil advantage—40-60% more area—vastly outweighs the negligible turn loss.
Complete System: The Pressure Budget
Let’s work through a complete integrated fairing-condenser system to demonstrate the engineering feasibility.
Reference Vehicle: 7-Ton Delivery Truck
Specifications:
- Truck width: 2.0m
- Usable width (between chassis rails): 1.7m
- Fairing depth: 0.5m
- Available height for equipment: 0.35m
Component Sizing
| Component | Dimension | Notes |
|---|---|---|
| Inlet | 0.25m H × 1.4m W = 0.35 m² | Across fairing face |
| Turn plenum | 0.3m depth × 0.25m height | 150mm corner radius |
| Condenser | 1.6m W × 0.5m D = 0.80 m² | Horizontal orientation |
| Fan plenum | 1.6m W × 0.15m H | Sealed to coil perimeter |
| Fans | 2× 280mm diameter | 100W each, angled 25° |
| Cowlings | 0.3m extension below fans | Directional exhaust |
Airflow Budget
Design flow rate: 1.2 m³/s (both fans combined)
Inlet velocity: 1.2 m³/s ÷ 0.35 m² = 3.4 m/s
Coil face velocity: 1.2 m³/s ÷ 0.80 m² = 1.5 m/s
Coil face velocity of 1.5 m/s is excellent—well below the 2.5-3.0 m/s that causes excessive pressure drop. The oversized coil provides efficiency benefits throughout the system.
Pressure Budget at 80 km/h
| Element | ΔP (Pa) | Notes |
|---|---|---|
| Available (inlet to exhaust) | +384 | Fairing face to side underbody |
| Inlet contraction | -5 | Minor acceleration loss |
| 90° turn | -3 | Radiused plenum design |
| Diffuser (inlet to coil) | -2 | 7° expansion angle |
| Condenser coil | -25 | 0.80 m² at 1.5 m/s |
| Fan plenum | -3 | Flow distribution |
| Cowling/exhaust | -8 | Directional outlet |
| Total system losses | -46 | |
| Net available | +338 | Assists fan operation |
At 80 km/h, the pressure differential nearly drives the required airflow without fan assistance. The fans supplement rather than solely generate the airflow.
Pressure Budget at Stationary
| Element | ΔP (Pa) | Notes |
|---|---|---|
| Available (inlet to exhaust) | 0 | No vehicle motion |
| Buoyancy opposition | -15 | Conservative estimate for downward exhaust |
| System losses | -46 | Same as above |
| Required fan pressure | ~61 Pa | Fans rated 50-80 Pa each |
The fans comfortably handle stationary operation. With two fans, there’s significant headroom—allowing reduced speed operation (quieter, more efficient) while maintaining adequate airflow.
Compare to Open-Mount Installation
Same condenser, same fans, open mounting:
| Element | ΔP (Pa) | Notes |
|---|---|---|
| Available (pressure differential) | ~0 | Open to open, no differential |
| Capture efficiency penalty | -70% | Only 25-30% of theoretical flow |
| Recirculation penalty | -15% | Crosswind conditions |
| Effective airflow | ~35% | Compared to rated capacity |
The open-mount system delivers roughly one-third the effective airflow of the integrated design. Same components, dramatically different performance.
Inlet Design Principles for Fairing Integration
The inlet is where all the pressure comes from. Get it right, and the system works efficiently. Get it wrong, and no amount of downstream optimisation will compensate.
Location: High-Pressure Zone
The fairing front face is the natural location—it’s where vehicle motion creates maximum stagnation pressure. But within this zone, there are better and worse positions.
- Optimal: Upper portion of fairing face, where clean airflow from over the cab meets the surface. This air has had minimal interaction with the cab body, minimal boundary layer development, maximum energy.
- Acceptable: Middle portion of fairing face. Slightly lower pressure than upper zone but easier to package with horizontal coil below.
- Avoid: Lower portion of fairing face, where boundary layer from cab sides creates disturbed, lower-energy flow.
Geometry: Rounded Edges
Sharp-edged inlets cause flow separation. Air hitting a sharp corner creates turbulence, recirculation, and reduced effective capture.
- Minimum edge radius: 20-30mm (0.1 × inlet height)
- Preferred: Full bell-mouth profile if space permits
Area Sizing
Inlet area depends on required mass flow and acceptable inlet velocity.
Target inlet velocity: 3-5 m/s
Higher velocities mean smaller inlets (packaging benefit) but higher losses. Lower velocities mean larger inlets but more gentle acceleration into the system.
A_inlet = Q / V_inlet
For 1.2 m³/s at 4 m/s:
A_inlet = 1.2 / 4 = 0.30 m²
Integration with Fairing Aerodynamics
The inlet opening is a discontinuity in the fairing surface. It creates some drag penalty unavoidably. Minimise this by:
- Smooth transitions: No protruding scoops or sharp edges
- Flush mounting: Inlet aperture recessed slightly into fairing surface
- Contoured surrounds: Guide approaching flow into inlet rather than around it
- Size appropriately: Larger-than-necessary inlets waste aerodynamic efficiency; smaller-than-necessary inlets starve the system
Exhaust and Cowling Design for Angled Fans
The exhaust path is where the pressure differential completes. Proper design ensures the negative pressure at the underbody is actually captured and utilised.
Cowling Functions
The cowling around each fan serves multiple purposes:
1. Bell-mouth inlet to fan: The transition from plenum to fan should be radiused, not sharp. This allows the fan to operate at better efficiency—typically 10-15% improvement versus sharp-edged mounting.
2. Directional exhaust: Guide the exhaust stream to the target zone (side underbody at 25° from vertical).
3. Weather protection: Prevent rain, road spray, and debris from entering the fan and condenser from below.
4. Structural support: Mount the fan motor securely while managing vibration.
Cowling Geometry
Cross-Section of Cowling:
══════════════════ ← Coil face (horizontal)
│
┌──────┴──────┐
│ Bell-mouth │ ← Radiused entry
│ ╱ ╲ │
└──╱ ● ╲──┘ ← Fan motor
╱ (fan) ╲
╱ ↓↓↓ ╲ ← Airflow direction
╱ ╲
╱ COWLING ╲
╱ (smooth inner ╲
╱ surface) ╲
▼ ▼
25° angle 25° angle
Design Features
- Inner surface: Smooth, continuous curve from fan exit to cowling outlet. No steps, gaps, or sharp corners that would cause separation or turbulence.
- Outer surface: Streamlined for vehicle motion, but secondary to inner surface function.
- Exit orientation: Pointing downward and outward. The exit plane should not face forward (would scoop road spray) or directly down (would face standing water).
- Drainage: Small channels or slots at the lowest points to allow any water ingress to escape without pooling.
- Guards: Mesh or louvers at the exit to prevent debris ingestion from below. Must be sized to avoid significant flow restriction (open area >70% of exit area).
Exit Area Sizing
The exhaust exit should be large enough to avoid creating a restriction that wastes pressure.
- Minimum exit area: 25% of inlet area (allows pressure recovery in diffusing exhaust)
- Recommended: 40-50% of inlet area
For an inlet of 0.35 m², total exhaust area (both cowlings combined) should be at least 0.14-0.18 m².
Scaling for Different Vehicle Sizes
The principles we’ve discussed apply across vehicle sizes, but the specific dimensions scale with cooling requirements.
Cooling Load by Vehicle Class
| Vehicle Class | GVW | Typical Cargo Volume | Cooling Load | Min. Condenser Area |
|---|---|---|---|---|
| LCV | 3.5t | 8-12 m³ | 2.5-4 kW | 0.25-0.35 m² |
| Light | 5-6t | 15-20 m³ | 4-6 kW | 0.35-0.50 m² |
| Medium | 7-8t | 20-28 m³ | 5-8 kW | 0.50-0.70 m² |
Scaling Rule of Thumb
Condenser area ≈ 0.02-0.025 × Cargo volume (m²)
A 20 m³ loadbox requires approximately 0.40-0.50 m² condenser face area for proper heat rejection at altitude with multi-stop duty cycles.
Fan Scaling
For dual-fan configurations:
| Condenser Area | Total Airflow | Fan Size (each) | Motor Power (each) |
|---|---|---|---|
| 0.35 m² | 0.7 m³/s | 220mm | 60W |
| 0.50 m² | 1.0 m³/s | 280mm | 100W |
| 0.70 m² | 1.4 m³/s | 320mm | 150W |
Altitude Correction
These specifications assume sea-level conditions. For Johannesburg operations at 1,750m elevation, apply the altitude correction factor discussed in our Technical Formulas Reference.
Capacity_altitude = Capacity_sea-level × (1 - 0.12 × Altitude_km)
Capacity_1750m = Capacity_sea-level × 0.79
Equipment should be specified 25-30% oversized compared to sea-level requirements to ensure adequate performance at altitude.
A Challenge to the Industry
We’ve presented calculations. We’ve shown our work. We’ve demonstrated—with physics, not opinion—why integrated ducted condenser designs outperform open-mount installations.
The transport refrigeration industry has repeated the same myths for decades. “Open air is better.” “Ram air helps.” “Enclosures trap heat.”
We challenge any installer, any manufacturer, any industry body to show their calculations.
- What capture efficiency does your open-mount installation achieve? Measure it. Calculate it. Prove it.
- What is the effective pressure differential in your system? At highway speed? At delivery stops? Quantify it.
- How does your system perform in a 20 km/h crosswind at stationary? What’s your recirculation fraction?
- What mass flow rate does your condenser actually receive versus its rated assumption?
We suspect these questions haven’t been asked because the answers would be embarrassing. It’s easier to repeat folklore than to do engineering.
Racing teams did the engineering. Automotive companies did the engineering. Every modern car has a ducted cooling system because the physics demands it.
Transport refrigeration is not exempt from physics. The refrigerant doesn’t care about industry tradition. The heat doesn’t care what installers find convenient. The thermodynamics is identical whether you’re cooling a Formula 1 power unit or a loadbox full of frozen vegetables.
The only question is whether the South African transport refrigeration industry will adopt engineering analysis or continue selling inferior solutions because they’re easier to install.
We’ve shown what’s possible. We’ve explained why it works. We’ve even provided the formulas so anyone can verify our claims.
Now it’s your move.
Conclusion: What Physics Demands
Let’s summarise what the calculations reveal:
- The ram air contribution is minimal. At typical courier operating speeds, ram air provides 11 Pa average versus 50-100 Pa from the fan. The fan does 85-90% of the work. Building your system around “ram air” optimises for 10-15% of your thermal management capacity.
- Capture efficiency is the hidden multiplier. Open mounts achieve 25% capture; ducted systems achieve 80%. Same condenser, same fan, three times more air through the coil with ducting.
- Exhaust direction matters. Upward exhaust wastes the underbody negative pressure (100+ Pa at highway speed). Downward exhaust captures it. Angled exhaust to side underbody captures even more.
- Recirculation destroys open-mount performance. 20-40% recirculation in adverse conditions reduces capacity by 10-20%. Ducted systems maintain near-zero recirculation in all conditions.
- Horizontal coil orientation maximises capacity. Within a fairing envelope, horizontal mounting delivers 40-60% more condenser area than vertical orientation. The 90° turn costs 2-3 Pa—less than 1% of available pressure differential.
- Integration beats bolt-on. Fairing + condenser as a single engineered system outperforms fairing + condenser as separate components that compromise each other.
These aren’t opinions. They’re calculations anyone can verify. The formulas are public. The physics is settled.
Every racing team encloses their coolers because the engineering demands it.
Your TRU should too.
For detailed calculations, worked examples, and formulas supporting this analysis, visit our Technical Formulas Reference.
Related reading
- The Aerodynamics Tax: R10,000-R70,000 Your Customers Pay Every Year
- Your Truck is a 4-Ton Brick: The Aerodynamic Design Revolution
- The Aerodynamic Cost of Larger Condensers: The Fairing Effect
The Frozen Food Courier operates temperature-controlled logistics across Gauteng and Western Cape. Our technical content draws from 8+ years of operational experience having driven ourselves to the moon and back twice, taking notes, because physics doesn’t care about marketing claims.
