How to calculate Pump Head in HVAC?

     In this post we will see how to calculate pump head in HVAC..

1. Calculating Pump Head in HVAC

Pump Head (Horsepower x Pressure) / Efficiency

Efficiency is the amount of water that can be moved per horsepower hour

For example if we have a 4 HP pump at 100 CFM(cubic feet per minute)

and it has an efficiency rating of 80% then we would say that the pump has a 4 hp/hr for an hour.

The formula is Horse Power x Pressure/(efficiency)

In hvac the horse power is measured in watts.

So we multiply our pressure(CFM) x watts HP & efficiency 60%

Example 2-

If you had a 1 HP motor that was running at 2000 RPM(revolutions per minute),

you could calculate how much pressure should be applied to the fan blades using the above equation.

2000 RPM x 60%1200 CFM Pressure

Now we know how many CFM the motor can deliver, we need to find out how much pressure the unit is applying to the fan blades.

Calculating Pump Head

Pump heads work by pushing water out of the system where they are located at the end of a line. The size of the pipe/line is called the diameter. A larger diameter pipe would have a higher flow rate than a smaller diameter pipe. Pressure and temperature play a role in how much water comes out of the system. Therefore, higher pressure requires less force to push water out. Higher temperatures require more force to move the liquid through the pipes. So while a line with a lower diameter will produce more water, it will also require more force to push the liquid through the pipes compared to a line with a higher diameter.

To calculate the volume of fluid per hour, take the formula V Q / P where V is volume (in gallons), Q is flow rate (gallons per minute) and P is pressure (pounds per square inch). Let's say we want to know how many gallons per hour our garden hose produces. We'd plug those numbers into the equation and get VHQ/P. To convert gallons to liters multiply by.45. So if the volume of water produced by the hose is 60 gallons per hour, then the flow rate is 1 liter per second. Since 1 liter 1000 milliliters, and 1 ml .001 gallons, we get 60 ml 6000 ml. Plugging those numbers into the equation gives us 12000 ml *.45 5400 ml per hour, which means that over twelve hours, our garden hose will produce 6 liters of water.

So in order to find out how many pounds per square inch it takes to produce 60 liters per hour, divide 60 by 0.45. That means we need approximately 11.11 PSI of pressure to produce 60 liters of water per hour.

If you're wondering what PSI stands for, it's actually pressure times one-over-pi squared.

*Note - If the unit of measure is not stated in the question, you'll need to use the conversion factor provided.

Here is another side on how to calculate the pump head :

The calculation of pump head in HVAC system is the same as calculating the pressure head in hydro systems. If we assume that the pump flow rate equals the water demand at the inlet, then the difference between the measured water level at the outlet (at the tank) and the water level at the inlet (at the suction pipe) is equal to the pump head.

As a general rule, 1 inch of rise means 0.75 psi of hydraulic head. However, the actual value depends on many factors including pipe diameter, length, material, etc. If the pipe diameter is smaller than 2 inches, then the relationship is different and is given by the following equation.

H 1/2πrho*(2x)^2Lp*Q

Where

H Pump Head

ρ Density of water

r radius of the pipe

x distance from the centerline of the pipe to the discharge point

Lp Length of Pipe

Q Flowrate of Water

In case the flow rate increases, the hydraulic head decreases and vice versa.

If the pipe diameter is greater than 2 inches, then it increases linearly with increasing length of pipe and decreasing flow rates.

1. Chilled Water Pump Head (CHWP)

The calculation of CHWP is based on the temperature increase caused by pumping water at the rate of flow over the same period of time at a higher temperature. In order to calculate the CHWP, it is necessary to know the following factors:

- Average Temperature (°F)

- Rate of Flow (US GPM)

- Pressure Loss Due to Pipe Diameter (PSI)

To determine the average temperature of the fluid entering the cooling coil, multiply the number of gallons per minute flowing into the coil by the ambient air temperature; then divide the result by the number of hours for which the measurement was taken.

Example: If the ambient air temperature at a particular location was 20°F and if a gallon of water flows into the coil every hour for an eight hour period then the average temperature is calculated as follows.

20 x 8 = 160

160 ÷ 8 = 20 °F

This is the average temperature of the coolant entering the cooling coil.

To determine the pressure loss due to pipe diameter, take the square root of the diameter of the pipe multiplied by the flow rate. To find the pressure drop across a 1/8 inch copper tube or line, use the following formula:

Pressure Drop Square Root Of Tube Diameter x Flow Rate

Example: A 1/8 inch copper tubing has a cross sectional area of 0.0625 inches squared, and a flow rate of 100 gpm. Then the pressure drop caused by the tube is equal to 4.30 psi.

1. What is the purpose of calculating chilled water pump head?

Chilled water pump head (also known as chilled water pressure) is the height at which the water is forced out of the chiller. Chilled water pressure is measured in feet above sea level.

2. How do I calculate chillers chilled water pump heads?

To calculate chilled water pressure, you need to know the following:

• The total volume of the incoming water

• Total flow rate of the incoming fluid

• Amount of heat added to the incoming water

• Average boiling point of the incoming water

3. How does chilled water pump head affect chiller performance?

As chilled water pump head increases, the chiller performance decreases. A high chilled water pressure may cause the coil fins to get damaged due to excessive force exerted onto the coils. Also, the amount of cooling provided to the load will depend upon the chilled water pressure and the size of the air handler. If the chilled water pressure is too low, then there won't be enough airflow passing over the coils, hence not enough cooling. On the other hand if the chilled water pressure is higher than required, then the excess pressure applied to the coil fins can damage them.

4. Why should my chilled water pressure be maintained at minimum?

If the chilled water pressure is lower than the recommended value, then there might be insufficient cooling. Such condition would lead to overheating of the refrigeration system components and loss of efficiency of the chiller. In addition, the reliability of the compressor could be affected.

5. How do I calculate chilled water head?

You can calculate chilled water pressure by multiplying the total quantity of water entering the chiller with the total volume of the chiller. Then divide the resulting number by the average temperature of the water entering the chiller. Thus you'll find the chilled water head.

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