How to size a cooling tower pump

In the cooling tower system, the circulating water pump (also known as the cooling tower pump or circulating pump) is the core equipment that ensures efficient dissipation of heat load. Proper selection of circulating water pumps can not only ensure stable operation of the cooling tower system, but also significantly reduce energy consumption and maintenance costs. This article will comprehensively introduce how to correctly select cooling tower circulating water pumps from principles, key parameters, calculation steps to selection considerations.

The function of cooling tower circulating water pump

The main function of the cooling tower circulating water pump is to transport the high-temperature water that absorbs heat from equipment (such as air conditioning units, condensers, process heat exchangers, etc.) to the cooling tower, and then return it to the equipment after heat dissipation, thus forming a stable closed-loop cooling circuit.

A well-designed water pump selection should meet the following requirements:

1. Provide sufficient traffic (Flow)

2. Provide sufficient head

3. Maintain normal pressure difference in the system

4. Possess high efficiency and low energy consumption

Three core parameters for selecting cooling tower pumps

1. Flow rate of water pump (unit: m ³/h or L/s)

Flow rate is the most critical parameter for selecting cooling tower pumps, which can generally be determined through the following methods:

(1) According to the cold ton (RT) calculation:

Common experience points:

Industrial cooling tower: 1 RT ≈ 0.27-0.32 m ³/h

Central air conditioning cooling tower: 1 RT ≈ 0.3 m ³/h (commonly used)

For example, a 500RT cooling tower

Flow rate=500 × 0.3=150 m ³/h

(2) According to the requirements of heat exchange equipment:

If the equipment manufacturer provides a rated flow rate, the equipment flow rate should be used as the standard.

2. Pump head (Head, unit: m)

The head of the water pump needs to overcome the following resistance:

1. System static pressure difference (if there is a high-level cooling tower, the building height should be considered)

2. Friction loss in pipelines (straight pipes, elbows, tees, valves, etc.)

3. Spray resistance of cooling tower (generally 3-8m)

4. Condenser resistance (usually 30-60kPa ≈ 3-6m, but data needs to be checked)

5. Safety margin (10% -20%)

3. Pump efficiency and power (kW)

The higher the efficiency of the water pump, the lower the operating cost.

When selecting, the appropriate model and motor power are usually determined based on the manufacturer's sample.

Calculation steps for selecting cooling tower water pumps (detailed example)

Example condition:

1.Cooling tower: 600RT

2.Total length of pipeline: 80 meters

3.Piping: DN150

4.Cooling tower resistance: 5 meters

5.Condenser resistance: 45 kPa (≈ 4.5 m)

6.Height difference: 5 meters

7.Friction loss of pipeline: 12 meters

8.Safety margin: 10%

Step 1: Calculate the flow rate of the water pump

Flow rate=Cold ton × 0.3

= 600 × 0.3

= 180 m³/h

Step 2: Calculate the head of the water pump

Total head=height difference+pipeline friction loss+cooling tower resistance+condenser resistance

= 5 + 12 + 5 + 4.5

= 26.5 m

Add a 10% safety margin:26.5 × 1.1=approximately 29 meters

Therefore, the selected lift should not be less than:

29 meters (taking 30 meters)

Step 3: Select the pump model and power

Based on the manufacturer's curve, select:

1.Flow rate: 180 m ³/h

2.Lift: 30 meters

3.Power: Approximately 15-22 kW (slightly different manufacturers)

Precautions for Selection of Cooling Tower Circulating Pump

1. The cooling tower is an "open system" and needs to consider the NPSH (net cavitation allowance)

High water temperature (32-37 ° C) can easily cause cavitation, and sufficient NPSHa should be left.

2. Pipeline design should avoid air accumulation

Avoid "air blockage" affecting the flow rate.

3. The water pump must be matched with the cooling tower

For example, the resistance of counter current tower and cross current tower is different.

4. Leave room for future expansion

If the system may increase its cooling capacity in the future, a slightly higher flow rate should be selected.

5. When multiple pumps are connected in parallel, attention should be paid to hydraulic balance

It is recommended to use dual or triple pumps in parallel to improve regulation efficiency.