Cooling Coil / Heat Exchanger of Carrier Brand AHU

Cooling Coil / Heat Exchanger of Carrier Brand AHU

Types of Cooling Coils in AHUs

Coils are categorized based on the cooling medium and design:

1. Chilled Water Coils

Medium: Circulates chilled water (typically 4–12°C) from a central chiller plant.

Design: Tubes with fins; water flows through tubes, absorbing heat from air.

Advantages: Easy to control (via water flow rate), lower pressure requirements, suitable for large AHUs.

Disadvantages: Dependent on a chiller plant; risk of freezing if water temperature drops too low.

2. Direct Expansion (DX) Coils

Medium: Uses refrigerant (e.g., R410A, R32, or low-GWP alternatives like R290) that evaporates directly within the coil.

Design: Similar fin-tube structure, but refrigerant expands and absorbs heat as it flows through the coil (connected to a compressor and condenser).

Advantages: Higher cooling efficiency (no intermediate water loop), faster response to temperature changes, ideal for smaller AHUs or decentralized systems.

Disadvantages: More complex refrigerant piping; requires precise charge control.

3. Steam/Hot Water Coils (for heating)

Medium: Steam or hot water (for heating mode).

Function: Releases heat to warm the air stream, often used in AHUs for year-round climate control.

Design and Structure

A typical AHU cooling coil consists of the following components:

Tubes: Small-diameter metal tubes (usually copper, aluminum, or stainless steel) through which the cooling medium (refrigerant or chilled water) flows. Copper is preferred for its high thermal conductivity, while aluminum is lighter and cost-effective.

Fins: Thin, closely spaced metal fins (aluminum is common) attached to the tubes. Fins significantly increase the surface area for heat transfer between the air and the tube’s contents, boosting efficiency.

Headers: Manifolds at the ends of the tubes that distribute the cooling medium evenly into the tube bundle and collect it after heat exchange.

Drain Pan: A shallow pan located beneath the coil to catch and channel away condensed moisture (critical for preventing water damage and mold growth).

Core Functions

Cooling Air:

The primary role is to remove heat from the incoming air. A cold medium (refrigerant or chilled water) circulates through the coil, absorbing heat from the warm air passing over its surface, thereby lowering the air temperature.

Dehumidification:

As air cools below its dew point, moisture condenses on the coil’s surface. This condensed water (condensate) is collected in a drain pan and removed, reducing the air’s humidity—essential for maintaining comfortable or process-specific humidity levels.

Heating (in dual-purpose coils):

Some coils are designed to handle hot water or steam, allowing them to reverse function as heating coils, raising air temperature when needed (e.g., in winter or industrial processes).