---
title: "Airflow Requirements for L2A CDU Explained"
description: "Learn how airflow affects L2A CDU performance, including the impact of air density and temperature rise on heat management in data centers."
image: "/media/i/a912e6b4-8623-43ce-8c3b-f2a9c4d1d8d9/how-much-airflow-does-an-l2a-cdu-need.webp"
---

L2A CDU airflow answers two questions: how much air the heat needs, and how much the unit delivers. The heat balance gives the first, and the coil, the fan curve and air density give the second.

The parent article covers this in airside capacity limits in a real hall: the heat the unit rejects ends up in the room's air, and the room absorbs only so much at its air conditions. This page covers what moves the airflow figure, the condition each number carries, and what the coil and fan deliver.

## Airflow From the Airside Heat Balance

Airflow in a liquid-to-air CDU carries the heat from the coil into the hall, so the figure a project needs follows the heat load and the state of the air being moved.

### The Air That Carries the Heat

CFM, cubic feet per minute, counts the volume of air a unit moves each minute. On its own it is only a number, and what that volume achieves depends on the air state at the site.

Air thins as it warms and at altitude, and thinner air carries less heat in the same volume. The airflow a duty needs rises with the heat load, and again as the air thins.

### The Permitted Rise

The permitted rise is chosen for the coil and the room alike. A small rise moves more air and a large rise moves less, and the room has to accept the discharge temperature either way.

That gives the airflow needed to carry the heat at that rise. It does not show that the coil reaches the duty, that the fan delivers the flow, or that the liquid supply holds its temperature.

A permitted rise leaves the coil area undecided because changing the airflow moves the airside coefficient and the air temperature distribution in opposite directions. The area is settled by a coil check, where the selected rise, the airflow and the coil performance data show whether the duty is reachable and what face area it takes.

## CFM per kW and the Air Temperature Rise

The ratio of airflow to capacity only means something once the air temperature rise behind it is stated.

### The Condition Behind the Ratio

Gerchamp's [CDU4175 L2A 175kW](https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW) ([Agent MD](https://www.gerchamp.com/en-US/products/liquid-to-air-cooling-distribution-unit-175kW/raw.md)) publishes 175 kW at ATD 10, 22,000 CFM and a 35 °C ambient. The three describe one product at one condition.

A figure in CFM per kW holds only for a stated air temperature rise. The airflow a site needs and the duty the unit delivers there follow that site's air state and permitted rise. The conditions depend on the project. For details, [contact us](https://www.gerchamp.com/en-US/contact) ([Agent MD](https://www.gerchamp.com/en-US/contact/raw.md)).

ATD, the approach temperature difference, is defined on liquid and air reference temperatures. It describes the gap the exchanger works across, and the air rise describes the airside path.

### The Ratio and Its Limits

A unit with a small permitted rise needs more air per kilowatt than one with a large rise. Two datasheets can sit far apart and both be right, since the ratio follows the rise.

The ratio therefore describes one operating condition. It cannot rank two products, and it does not set a norm for a power class.

Density sits inside the ratio too: the same volume holds less mass at altitude, and carries less heat.

## Coil Face Velocity and Its Cost

Face velocity links the airflow to the coil, and it is calculated on the coil's own face area.

### Face Area Against Velocity

Face velocity is volume flow divided by the actual coil face area, which is the open face of the coil. The unit footprint and the cabinet opening are different numbers, and neither can stand in for the coil face area.

A larger face slows the air for the same flow, a smaller face speeds it up. Within a similar coil construction and operating range, a higher velocity raises the airside coefficient and allows a more compact coil.

### The Cost of Higher Velocity

Airside pressure drop rises with face velocity, and in most coil constructions it rises faster than the velocity does. Fan power follows the system resistance, the fan efficiency and the operating point the fan settles at, and the selection has to carry that cost.

Water carryover belongs to a coil whose surface sits below the dew point of the air passing it. A liquid-to-air unit runs dry in normal operation, so that limit enters on a cold start or an abnormal low-temperature event where condensate forms.

## Fan Static Pressure and Air Density

The fan has to overcome the total resistance along the air path from the intake to the room.

### System Resistance and the Fan Curve

Coil, louvre, grille, any ductwork and any air filter each add resistance, and the fan has to work against their sum. The point where the fan performance curve meets this total resistance sets the airflow the unit delivers.

Where the air path includes a filter, dust loading adds resistance and pushes the delivered airflow down. Where it does not, coil fouling, a blocked grille or added ductwork do the same job.

Fan control decides how the unit answers that resistance. A fixed-speed fan follows its curve and loses flow as the filter loads. A constant-airflow control adds speed to hold its setpoint until it reaches its limit. A constant-pressure control holds pressure at its sensing point, and that does not by itself fix the total flow.

Static pressure, total pressure and system pressure drop are three different quantities. A fan curve and a system curve compare only on one basis and at one measuring plane.

### Air Density and Site Conditions

Density falls as the air warms and at altitude. At a given speed the fan keeps moving roughly the same volume of air, and in thinner air each cubic metre carries less mass, so the heat leaving with the air falls at the same rise. What a fan delivers in thinner air follows its curve and the resistance it works against.

The correction applies where the site condition differs from the rated condition. A unit rated at a stated ambient already carries that condition inside its figure, so density adds no second derate for the same 35 °C. At altitude or on a hotter intake, the airflow and its duty follow the site. The conditions depend on the project. For details, [contact us](https://www.gerchamp.com/en-US/contact) ([Agent MD](https://www.gerchamp.com/en-US/contact/raw.md)).

## Frequently Asked Questions

**Does more airflow always give more capacity?**
No. Extra air carries a smaller rise, and the coil and liquid side decide whether the duty is reachable at all.

**Is there a normal CFM per kW for this class?**
No single figure. The ratio follows the permitted rise, so a quote with airflow alone says little.

**Why does altitude matter for airflow?**
Pressure falls, each cubic metre holds less mass, and heat removal follows mass.

**Can I cut airflow to save fan energy?**
Only where the coil still reaches the duty at the larger rise and the room absorbs the warmer discharge.

## The Bottom Line

L2A CDU airflow starts with the heat a hall has to shed and the air that carries it. More heat calls for more air, a wider rise calls for less, and thin air at altitude calls for more volume for the same duty.

What the unit then delivers comes from elsewhere: the coil check for duty, the fan curve against system resistance, the density at the site, and the control mode. The requirement and the delivery are two separate questions.

---

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