How to calculate the cooling load of an open circuit cooling tower?

Calculating the cooling load of an open circuit cooling tower is a crucial step in ensuring its efficient operation and proper sizing for industrial or commercial applications. As a supplier of open circuit cooling towers, I understand the importance of accurate cooling load calculations to provide our customers with the most suitable and cost - effective solutions. In this blog, I will guide you through the process of calculating the cooling load of an open circuit cooling tower.

Understanding the Basics of an Open Circuit Cooling Tower

An open circuit cooling tower operates by bringing warm water into contact with air. The heat from the water is transferred to the air through evaporation and sensible heat transfer. This process cools the water, which can then be recirculated back to the industrial process or equipment that generates heat. There are different types of open circuit cooling towers available, such as Cross Flow Open Type Cooling Tower, Cross Flow Open Loop Cooling Tower, and Cross Flow Steel Open Cooling Tower. Each type has its own characteristics and is suitable for different applications.

Factors Affecting the Cooling Load

Before diving into the calculation, it's essential to understand the factors that affect the cooling load of an open circuit cooling tower.

Heat Load from the Process

The primary source of heat in a cooling tower is the heat generated by the industrial process or equipment. This could be from a chiller, a power generation unit, or a manufacturing process. The heat load from the process is usually measured in British Thermal Units per hour (BTU/h) or kilowatts (kW). To determine this heat load, you need to know the power consumption of the equipment, the efficiency of the process, and the amount of heat generated per unit of work.

Ambient Conditions

The ambient temperature, humidity, and air velocity play a significant role in the cooling tower's performance. Higher ambient temperatures and humidity levels reduce the cooling tower's efficiency because the air has less capacity to absorb heat and moisture. Air velocity affects the rate of heat transfer and evaporation in the cooling tower. In general, higher air velocities can enhance the cooling process, but excessive velocities may cause water drift.

Water Flow Rate

The flow rate of water through the cooling tower is another important factor. A higher water flow rate means more heat needs to be removed, increasing the cooling load. However, a very high flow rate may not allow sufficient contact time between the water and air, reducing the cooling efficiency.

Calculation Methods

Step 1: Determine the Heat Load from the Process

The heat load from the process ($Q_p$) can be calculated using the following formula:

If the power consumption of the equipment ($P$) is known in kilowatts, and the efficiency of the process ($\eta$) is given, the heat load in kilowatts is:

$Q_p=P\times(1 - \eta)$

To convert the heat load from kilowatts to BTU/h, use the conversion factor: 1 kW = 3412.14 BTU/h

For example, if a chiller has a power consumption of 100 kW and an efficiency of 0.8, the heat load is:

$Q_p = 100\times(1 - 0.8)=20$ kW

In BTU/h, $Q_p=20\times3412.14 = 68242.8$ BTU/h

Step 2: Consider the Heat of Evaporation

In an open circuit cooling tower, a significant amount of heat is removed through evaporation. The heat of evaporation ($Q_e$) can be calculated using the following formula:

$Q_e = m\times h_{fg}$

where $m$ is the mass flow rate of water evaporated (in lb/h or kg/s), and $h_{fg}$ is the latent heat of vaporization of water. At standard atmospheric pressure, the latent heat of vaporization of water is approximately 970 BTU/lb or 2257 kJ/kg.

The mass flow rate of water evaporated can be estimated based on the cooling range ($\Delta T$) and the approach ($A$) of the cooling tower. The cooling range is the difference between the inlet and outlet water temperatures, and the approach is the difference between the outlet water temperature and the wet - bulb temperature of the ambient air.

A common approximation for the mass flow rate of water evaporated is:

$m=\frac{Q_p}{h_{fg}}$

Step 3: Account for Sensible Heat Transfer

In addition to the heat of evaporation, there is also sensible heat transfer between the water and air. The sensible heat transfer ($Q_s$) can be calculated using the formula:

$Q_s = m_w\times C_p\times\Delta T$

where $m_w$ is the mass flow rate of water through the cooling tower (in lb/h or kg/s), $C_p$ is the specific heat capacity of water (1 BTU/lb - °F or 4.18 kJ/kg - °C), and $\Delta T$ is the cooling range.

The total cooling load ($Q_{total}$) of the open circuit cooling tower is the sum of the heat load from the process, the heat of evaporation, and the sensible heat transfer:

$Q_{total}=Q_p + Q_e+Q_s$

Example Calculation

Let's assume we have an industrial process with a heat load of 500,000 BTU/h. The water flow rate through the cooling tower is 1000 gpm (gallons per minute). The inlet water temperature is 95°F, and the outlet water temperature is 85°F. The wet - bulb temperature of the ambient air is 75°F.

First, calculate the cooling range: $\Delta T=95 - 85 = 10$°F

The mass flow rate of water in lb/h:

1 gallon of water weighs approximately 8.34 lb. So, for a flow rate of 1000 gpm, the mass flow rate of water $m_w$ is:

$m_w=1000\times60\times8.34 = 500400$ lb/h

The sensible heat transfer:

$Q_s=m_w\times C_p\times\Delta T$

$Q_s = 500400\times1\times10=5004000$ BTU/h

The heat load from the process is $Q_p = 500000$ BTU/h

The heat of evaporation can be estimated. Assuming a simple approximation, we can calculate the mass of water evaporated based on the heat load from the process.

$m=\frac{Q_p}{h_{fg}}=\frac{500000}{970}\approx515.46$ lb/h

$Q_e=m\times h_{fg}=515.46\times970 = 500000$ BTU/h

The total cooling load:

$Q_{total}=Q_p+Q_e + Q_s=500000+500000 + 5004000=6004000$ BTU/h

Importance of Accurate Cooling Load Calculation

Accurate cooling load calculation is essential for several reasons. Firstly, it ensures that the cooling tower is properly sized. An undersized cooling tower will not be able to meet the cooling requirements, leading to overheating of the equipment and reduced efficiency. On the other hand, an oversized cooling tower will result in higher initial costs and increased energy consumption.

Secondly, it helps in optimizing the operation of the cooling tower. By understanding the cooling load, you can adjust the water flow rate, air flow rate, and other operating parameters to achieve the best performance and energy efficiency.

Contact for Procurement and Consultation

As an open circuit cooling tower supplier, we have the expertise and experience to help you accurately calculate the cooling load and select the most suitable cooling tower for your application. Whether you are in the power generation, manufacturing, or HVAC industry, our team of professionals can provide you with customized solutions. If you are interested in learning more about our products or need assistance with cooling load calculations, please feel free to contact us for procurement and consultation.

References

• ASHRAE Handbook - HVAC Systems and Equipment. American Society of Heating, Refrigerating and Air - Conditioning Engineers.
• Cooling Tower Institute. Technical Manuals and Guidelines for Cooling Tower Design and Operation.