Heating Ventilation and Air Conditioning Full Course

This is part 15th of the full HVAC course. You can access all the previous parts at the end of this post.

Comparison between Fan Coil Unit (FCU) and Air Handling Unit (AHU)

Fan Coil Unit (FCU):

* Capacity ranges up to 2000 cfm. i.e; 5 Tr to 7 Tr.

* Types of FCU - Decorative FCU, Concealed FCU, etc.

* Fan motor shaft is directly connected to the blower. i.e; directly driven.

* Direct throw of air and noise in the duct can erupt, and there is no use of the duct also.

* It is a low-capacity cooling machine.

Air Handling Unit (AHU):

* Capacity about 2000 cfm up to 100000 cfm. i.e; 250 Tr to 500 Tr

* Types of AHU (Concealed) - Ceiling suspended up to 25 Tr, Floor Mounted above 25 Tr.

* Blower is connected to motor with the help of belt/pulleys.

* Uses ducts for equal distribution of air.

* It is a high-capacity cooling machine.

Cooling Towers:

The cooling tower was originally designed for industrial applications where water had to be passed through and cooled before being reused. This was achieved by passing large volumes of air over the liquid at high velocity, thereby extracting heat from the fluid. As the industry evolved, this method proved useful in other applications, including domestic HVAC systems. In these cases, the system would use a similar configuration except that the volume of air would be smaller, along with its velocity, and the size of the towers would have to be scaled down accordingly.

The primary function of the cooling tower is to remove excess heat energy from the circulating water, reduce the temperature of the incoming water, and increase the efficiency of the overall system. The tower consists of three major components: evaporator, condenser, and fan coil assembly. The vapor generated by evaporation must travel across the surface area of the evaporator before reaching the condenser, otherwise, the hot water could cause corrosion damage to the equipment. Therefore, the design of the evaporator is extremely critical.

In cooling towers, water removes the heat of the condenser by using evaporative cooling. Water is exposed to the outside air to reject heat from the water. The cooling tower works for cooling high-temperature water/fluids. Example. A typical cooling tower cools at 95 °F entering water temperature or Hot water supply down to 85 °F hot water return (low water temperature).

Cooling towers are used in conjunction with water-cooled chillers. The difference between the Entering Water temperature and Leaving water temperature is termed a range and it is usually taken as 8 - 10 °F.

1. Industrial Cooling Towers

Industrial cooling towers are used to cool water that is circulated through a facility’s air-conditioning system. This helps keep the air circulating inside the building at comfortable temperatures during hot summer days. They have many fans around their perimeter, which blow air across coils.

The water runs through the tower at high speed. A difference in density between the cold water and the warm air causes the rising air stream to spiral outward. As the air rises, it eventually loses energy and falls back toward the ground. The falling air then hits the coil again causing the cycle to repeat.

2. Residential Cooling Towers

A residential cooling tower is similar in function to its industrial counterpart but is used to cool water that has been recirculated from an individual's home AC unit. Like a commercial tower, this type of cooling device uses a series of blades attached to a central hub. These blades create a vortex that pulls down outside air into the center of the tower. Once the air is cooled, it flows out the backside of the tower where it is expelled outside.

What are cooling towers?

Cooling towers use large volumes of water to cool air through evaporation. They are used inside power plants and other industrial facilities where they can provide a year-round supply of clean cold water. These towers are typically located near power generation units and serve to regulate the temperature of the air that enters them by evaporating a portion of the water vapor from the surrounding air. This condensed water collects at the bottom of the tower and flows back into the body of water nearby. Cooling towers contain numerous components, including a heat exchanger, fans, drain pans, control systems, and motors.

Where do these towers come from?

The first cooling towers were created in 1854 by Thomas Alva Edison. In the early days of electricity production, cooling towers were necessary to keep transformers operating safely and efficiently. They were designed to create a constant flow of cool water over the coils of electrical equipment, thus protecting the equipment from overheating. Today, cooling towers are still used in areas where electrical equipment generates enough heat to require cooling; however, their uses have expanded into many industries outside of traditional power generation.

How does this system work?

In order to understand how cooling towers work, we need to take a closer look at the basic structure of the heating and cooling cycles. A typical system consists of two major parts—the cooling coil and the condenser coil. At the top of the tower is the fan, which causes air to pass across the cooling coils. Once the air has been cooled by passing through the cooling coil, the air comes out of the bottom of the tower. The air then passes over the condenser coil, where it picks up its moisture content. The moisture from the air then falls back down onto the cooling coils, where it is once again evaporated. The cycle repeats until the air has passed through the cooling coils enough times to reach the desired temperature.

Types of Cooling Towers:

1. Natural Draft Cooling Tower :

For most natural draft cooling towers, Nozzle designs of water pressure drop of 7 - 10 psi (pound/ sq inch) (0.5 bar to 0.7 bar) will produce a suitable spray pattern of water sprinkling. Air movement causes evaporation which draws heat from the water. This works like a giant fan forcing cooler air over the water surface while moving warm air away. A draft cooling tower requires little maintenance, but regular cleaning is recommended.

2. Mechanical Draft type cooling tower :

Here in this, spray eliminators are used in mechanical types to prevent excessive drift losses. Generally, orifices are used in place of nozzles (as used in natural draft type) which reduces head on the water-circulating pump.

Drawbacks/disadvantages

- Cost of operation of the blower is more in Mechanical draft type.

- Wooden fill/ materials accumulate more dirt and sludge and create a problem for cleaning.

Mechanical Draft type cooling towers are classified into two types:

* Induced Draft Cooling tower

* Forced Draft Cooling tower

* Induced Draft Cooling tower

If a fan/blower induces air through the cooling tower, then it is known as an Induced Draft Cooling tower.

* Forced Draft Cooling tower

If a fan/blower forces air up through the tower, then it is known as a forced draft cooling tower.

3. Cross Flow cooling Tower

If hot water enters at the top and passes over fill/material, then air may enter either from one side (single flow) or from both sides (Double flow). Such type of cooling tower is known as crossflow cooling tower.

4. Counter Flow Cooling Tower

If hot water enters the top end flow downward or is drawn while air is introduced at the bottom and exits from the top of the phenomena, such type of cooling tower is known as a counter-flow draft cooling tower.