Genset room airflow decides whether the radiator can do the job it was selected for, because a core with no air behind it rejects nothing. Tesla Power sets out the four figures we check on a plant room, and shows what a site survey has to measure before a supplier can price a louvre.
- Start from the air volume the fan moves at the system resistance, not from the fan's free air figure.
- Convert louvre opening to free area, since 45% is the figure most banks achieve once the blades are counted.
- Give the outlet at least as much area as the inlet, because the discharge is hotter and expands.
- The frames themselves are listed in the range of frames the factory builds, and each data sheet names the air volume at the rating.
What does genset room airflow decide on a 550kVA set?
On a 550kVA set at 0.8 power factor the alternator delivers 440kW, and the fan moves about 11 cubic metres a second through the core at the system resistance. That figure sets the inlet and the outlet, and both have to pass 11 cubic metres a second without adding more than 0.12 bar to the fan.
Genset room airflow for standby duty with the doors closed is the case the room is really designed for. The radiator is the easy part of the order. What decides the room is the free area of the louvre bank, because a bank with 45% free area passes less than half of what its frame suggests. Yangzhou Tesla Power reads the opening on the drawing before the frame is released, since a louvre added later is a louvre sized on a guess. The room, not the radiator, is where a hot site loses its margin.
How is the air volume calculated from the heat?
Work back from the heat and the permitted rise. A room that accepts a 12 K rise through the space needs a volume that carries 350kW of heat away, and at 45 ℃ and 1000 m the air weighs 1.03 kg per cubic metre. Divide 350kW by the product of the mass flow, 1.005 kJ per kilogram kelvin and 12 K.
The result is 27.4 kilograms a second, which is 26.6 cubic metres a second of air through the room. Half of that passes the core and the rest sweeps the engine block and the alternator, and the fan only moves the core share. The room figure is the one the louvres are sized on, and it is roughly twice the fan figure on a set that rejects through the block as well as the core.
| Item | Figure at 550kVA | Where it comes from |
|---|---|---|
| Heat to the room | 350kW | Core plus block plus alternator |
| Permitted rise | 12 K | Site rule for a plant room |
| Mass flow | 27.4 kg/s | Heat over specific heat times rise |
| Volume flow | 26.6 m³/s | At 1.03 kg per cubic metre |
| Fan share | 11 m³/s | Data sheet at the rating |
Why does the louvre free area decide the room?
A louvre bank is a set of blades, and the air passes between them rather than through the frame. A bank with blades at 45 degrees and 100 mm pitch typically reaches 45% free area, and a bank with a bird screen and a sand trap drops to 35%. The frame dimension is therefore about twice the opening that matters.
Face velocity on the free area is the number to hold. Below 2.5 metres a second a louvre is quiet and adds little pressure drop; above 4 metres a second the blades start to whistle and the rain defence fails. Tesla Power sizes the opening from the room heat, and the room carries 350kW at 26.6 cubic metres a second, so 26.6 ÷ 2.5 = 10.6 square metres of free area holds the velocity at 2.5 metres a second. At 45% free area that is 10.6 ÷ 0.45 = 23.6 square metres of frame, which on a 550kVA set is a wall 2.4 m high and 10 m long.
How much outlet area does the discharge need?
Give the outlet the same free area as the inlet at minimum, and 1.2 times the inlet free area where the discharge turns a corner on its way out. The air leaves the radiator at about 55 ℃, so it is 4% less dense than the inlet air and carries more volume for the same mass.
A restricted outlet is worse than a restricted inlet, because it raises the pressure inside the room and the fan then works against the room as well as the core. We have measured 0.2 bar of room pressure on a plant room with a correctly sized inlet and a louvre door on the outlet, and the set ran 8 K hotter than its test sheet. The outlet is the half of the room most often drawn small, because it faces the yard.
Where does the inlet air come from on a hot site?
It comes from the coolest face of the building, and that is rarely the face the architect drew. A west wall at four in the afternoon radiates at 60 ℃ and the air against it sits 6 K above the yard. An inlet on that face starts the calculation 6 K behind, and the core then has 6 K less to work with.
The height matters too. Air at 2 m above a tarmac yard is 3 to 4 K warmer than air at 8 m, because the surface re-radiates. An inlet at high level on a shaded face is the cheapest cooling improvement on most sites, and it costs nothing at the drawing stage. A survey that records the face, the height and the afternoon temperature closes the question in a single visit to the site.
How does the room airflow change with altitude?
Altitude thins the air, so the same volume carries less mass and the room needs a larger opening for the same heat. At 1500 m the density is about 16% below sea level, which pushes the 26.6 cubic metres a second to 31.7 and the free area to 12.7 square metres of louvre.
The fan is affected twice. It moves the same volume at the same speed, so the core sees the same velocity but a lower mass flow, and the rise across the core climbs. That is why a high altitude site needs both a larger louvre and a larger core, and why one without the other runs hot. The two corrections multiply rather than add, which is the point most offers miss when a site sits above 1200 m.
What does an undersized louvre cost against a large one?
On a 550kVA set at 1500 m the correction comes to about 12% of the rating, so the set carries about 12% less than its plate figure. The pair belongs on the offer rather than the ambient alone, because a buyer who corrects one without the other still misses the target.
Tesla Power Equipment prices the two answers on one sheet. An undersized louvre costs the fan margin first and the coolant margin second. At 4.4 metres a second the bank adds 0.09 bar, the fan moves 8% less air, and the rise across the core climbs by about 9%. That is the difference between a set that settles at 88 ℃ and one that touches 95 ℃.
The larger louvre costs wall area and money, and it is the cheaper of the two answers. A bank sized at 2.5 metres a second on free area costs perhaps 20% more than one sized at 4 metres, and the fan power saved pays that back inside two years on a set that runs daily.
Where does genset room airflow go wrong on site?
The first error is a louvre sized on frame area rather than free area, which halves the opening without anyone noticing. The second is an outlet restricted to a door with a grille, which raises room pressure and pulls the fan off its curve. The third is an inlet on a sun-facing wall, which starts the calculation several degrees behind.
Both errors show in the first hot week rather than at commissioning, because commissioning is often done in a cooler month with the doors open. A room that passes at 30 ℃ ambient can fail at 45 ℃, and the record that proves the difference is the ambient beside the coolant figure.
Tesla Power quotes the fan volume rather than a free air figure, and most genset manufacturers quote the free air figure instead. A genset room airflow for the plant room is priced on the free area, not on the frame. Chinese genset suppliers often do the same, and the gap is where a set that was correct on the test bench runs hot in a plant room with a louvre door.
Which measurements belong in a room airflow survey?
Five measurements do it, and they are the same five for a genset room airflow in a basement plant room as on a roof. The free area of every inlet and outlet. The distance from the grille to the nearest wall. The height of the inlet above the yard. The temperature at the inlet at three in the afternoon. The temperature at the outlet with the set at 80% load.
With those five a supplier can check the room against the fan curve without visiting twice, and a buyer can see whether the room was built to the drawing. Tesla Power Equipment turns the five into a free area figure on the room sheet.
The 500kVA silent set with the ATS option ships with a room sheet that names the free area, and the 400kW frame on the Weichai WP13 platform carries the same sheet, and the the engineering notes on site planning set out the clearances for common buildings.
Which documents prove the room suits the set?
Three documents do it, and all three travel with the order. The room drawing carries the free area and the velocity it was sized to. The fan curve carries the system resistance including the louvre. The commissioning sheet carries the inlet and outlet temperatures at 80% load.
Conformity assessment for the assembly checks that the file matches the set rather than the quotation. The reference conditions behind the rating sit in the ISO 8528-1 rating definitions, and the test interval for a standby set follows the NFPA 110 emergency power rules.
Send the room drawing and the five measurements to our export desk, and the reply comes back as a free area figure with the velocity beside it. That is a room a contractor can build to, rather than a note that says the room must be ventilated.
How is the room checked after the louvres are fitted?
Run the set at 80% for two hours with the doors closed and log the inlet temperature, the outlet temperature and the coolant figure every fifteen minutes. A room that suits the set holds the inlet within 3 K of the outside figure and the coolant 8 to 12 K below the alarm.
A room that is short shows the inlet climbing through the second hour, because the discharge is recycling into the inlet. That pattern is a louvre problem rather than a core problem, and the fix is free area or a deflector rather than a larger radiator. A differential thermometer across the room tells the two apart in ten minutes.
Tesla Power checks the discharge louvre as well. An open louvre on that face is worth a look. Some banks are fitted with a blanking panel to keep rain out, and the panel is often left in place by a contractor who did not read the drawing. We have found three of those in five years, each on a set that had been reported as overheating. The same frame appears in the 1000kW container build for standby duty, where the plate states the same basis.
Frequently asked questions about genset room airflow
Size it on the air volume and hold the velocity on free area below 2.5 metres a second. On a 550kVA set that is about 26.6 cubic metres a second, which needs 10.6 square metres of free area, or a louvre frame of roughly 23 square metres at 45% free area.
It is fair for a bank with 45 degree blades at 100 mm pitch and no screen. Add a bird screen and the figure falls to about 35%, and add a sand trap and it falls further. Take the figure from the louvre maker rather than from the frame dimension.
It should be at least equal and 1.2 times the inlet where the discharge turns. The air leaves the core at about 55 ℃, so it is less dense and carries more volume. A restricted outlet raises room pressure and pulls the fan down its curve.
Because air at 2 m over a tarmac yard is 3 to 4 K warmer than air at 8 m, and a wall facing the afternoon sun adds more. A high level inlet on a shaded face is the cheapest improvement on most sites and costs nothing at the drawing stage.
It can be, in the sense that a large room holds a large volume of hot air that takes time to clear after a cold start. A room of 400 cubic metres on a 550kVA set clears in about 15 seconds at the fan volume, so the effect is small. The louvre area matters more than the volume.
About 0.03 bar at 2.5 metres a second on free area, and about 0.09 bar at 4.4 metres a second. The rise follows the square of the velocity, so a bank run at twice the design velocity costs four times the pressure and takes a visible bite out of the fan curve.
The inlet and outlet temperatures with the set at 80% load and the ambient noted beside them. A pair taken with the doors open proves nothing, and a pair taken in a cool month misleads. Tesla Power asks for the pair with the doors closed.
It needs the same calculation with a different free area figure, because a container louvre is smaller and the air turns inside the box. The louvre is sized on the fan volume at the box ambient, which runs 6 to 8 K above the yard, and the figure is printed beside the rating.