Chilled water systems circulate chilled water in a closed loop from chillers to air handling units and fan coil units. I've always wondered how to decide the pressure rating of valves and fittings. So, I decided to find out the closed loop chilled water system pressure.
Generally, most chilled water systems operate at a pressure of around 10-12 bar. Some chilled water systems work at more than 16 bar of pressure but they rarely exceed 20 bar. High pressure chilled water systems usually use heat exchangers to reduce the pressure to below 20 bar.
Components such as valves, fittings and cooling coils have a certain pressure rating. Typically, they are rated at either PN10, PN16, PN20 or PN25 which represent 10 bar, 16 bar, 20 bar and 25 bar respectively. The higher the pressure rating, the higher the initial cost.
|Pressure Rating||Maximum Working Pressure|
To know what pressure rating should be used, HVAC engineers often add the static pressure and the pump head to get the total pressure in a closed loop chilled water system. But, is that the right way to calculate it?
To find out how to calculate the total pressure in a closed loop chilled water system, I spent many hours researching Hydronics and pipes and the relationship between flow and pressure. Furthermore, I've spoken to a few HVAC seniors to understand the principle behind flow and pressure.
What is Pressure?
Pressure is the amount of force exerting on a surface. In a closed loop chilled water system, there are two types of pressure; a) static pressure and b) dynamic pressure. The sum of static pressure and dynamic pressure is stagnation pressure.
Static pressure is the internal pressure experience by pipes, valves, fittings and cooling coils when the chilled water is flowing, meaning when the chilled water pump is running.
The formula used to calculate static pressure is:
Ps = ρgh
Ps = Static pressure, Pa
ρ = Density of water, 1000 kg/m3
g = Gravity, 10 m/s2
h = Height, m
Static pressure is sometimes referred to as hydrostatic pressure.
If the chilled water is not moving, people often use hydrostatic pressure. If the chilled water is moving, they call it static pressure. Otherwise, it's hydrostatic pressure.
I know it's confusing. I think both static pressure and hydrostatic pressure are the same. It's just that people cut out the “hydro” part and shortened it to just “static”. Maybe I'm wrong. Anyway, it's not a big deal. I'll just use static pressure.
When a valve is opened halfway, its shaft is blocking some of the chilled water. This additional pressure exerting on the shaft is called dynamic pressure.
The formula used to calculate dynamic pressure is:
Pd = 0.5ρv2
Pd = Dynamic pressure, Pa
ρ = Density of water, 1000 kg/m3
v = Water velocity, m/s
Since water velocity is involved, dynamic pressure only occurs when the chilled water is moving. However, if the shaft of the valve gives away and flows together with the chilled water at the same velocity, the dynamic pressure is zero.
The sum of static pressure and dynamic pressure is stagnation pressure. Stagnation pressure is also known as total pressure and the formula used to calculate total pressure is:
Pt = Ps + Pd
Pt = ρgh + 0.5ρv2
Again, only moving chilled water has stagnation pressure on HVAC components. Meanwhile, stationary chilled water only experiences static or hydrostatic pressure.
Closed Loop Chilled Water System Pressure
In a closed loop chilled water system, pressure is always higher on lower floors than on upper floors due to static pressure. However, when the chilled water pump is running, should the system pressure be higher?
Static Pressure of Non-Moving Chilled Water
When the chilled water pump is not circulating the chilled water, there is a static pressure exerting on the internal surface of pipes, valves, fittings and cooling coils.
Below is a closed loop chilled water system diagram where the chilled water is not moving yet:
From the above diagram, the chilled water inside the pipe is pressing down harder on the gate valve and the AHU that is located on the lower floor than the chiller on the upper floor.
The pressure exerting on the gate valve is:
Ps = ρgh
Ps = (1000)(10)(70)
Ps = 700,000 Pa
Ps = 700 kPa or 7 bar or 101 psi
When the chilled water is not moving, the pressure exerting on the gate valve is 7 bar or 101 psi. So, the gate valve needs to have at least a pressure rating of PN10 in order to withstand the static pressure.
Total Pressure of Moving Chilled Water
When the chilled water is moving, there should be more pressure in the system by logic. I thought so but, it is more than that.
Below is a closed loop chilled water system diagram where the chilled water is moving now:
When the chilled water is flowing, there is still a static pressure exerting on the gate valve. The moving chilled water is exerting 7 bar of static pressure on the internal surface of the gate valve.
If we zoom in on the gate valve, we can see that the static pressure is exerting on the valve and the pipe in all directions as illustrated in the below diagram:
Now, what happens if the gate valve is partially closed? When the gate valve is partially closed, it will experience an additional dynamic pressure as illustrated in the below diagram:
When the gate valve is partially closed, part of the shaft gets “hit” by the moving chilled water. The chilled water “rubbing” through the shaft of the gate valve exerts a dynamic pressure on the gate valve.
To help you understand it better, imagine that you are inside a giant pipe filled with water. When you are in the water, you can feel that the water is exerting pressure on you from all directions. If you suddenly stop flowing with the water and stand still, you'll feel a dynamic pressure slamming onto your face.
Since most chilled water systems have a water velocity of 3 meters per second, the dynamic pressure exerting on the gate valve is:
Pd = 0.5ρv2
Pd = (0.5)(1000)(32)
Pd = 4500 Pa
Pd = 4.5 kPa or 0.045 bar or 0.65 psi
So, the total pressure exerting on the gate valve is:
Pt = Ps + Pd
Pt = 704.5 kPa or 7.045 bar or 101.65 psi
As you can see, the total pressure exerting on the gate valve is almost the same as the static pressure alone. Hence, we can say that the dynamic pressure is negligible.
So, it's true that moving chilled water has a higher pressure than non-moving chilled water. However, the difference is minor and sometimes, negligible.
Then, how about a fully closed gate valve? Will it experience greater pressure and thus, need a higher pressure rating? Well, let's find out from the below diagram:
By looking at the above diagram, we can imagine the gate valve as the pipe. When the gate valve is fully closed, the pressure is equivalent to when the chilled water is not moving which is Ps = ρgh again.
So, it seems like the pressure exerting on a fully closed gate valve is in fact smaller than the pressure exerting on a partially closed gate valve.
In short, the total pressure in a closed loop chilled water system is the static pressure and the static pressure is all about the height. But, what if you oversized the chilled water pump?
Pump Head vs Total Pressure
The purpose of the pump head is to ensure that the chilled water pump is able to move chilled water at the required flow rate. If the pump head is too small, the flow rate will not be enough. Conversely, if the pump head is too big, the flow rate will be too high.
In a closed loop piping system, the pump head is calculated by adding all pressure drops across valves, fittings, cooling coils and other HVAC components excluding the height.
So, if the pump head is oversized, the flow rate increased. Assuming the pipe size doesn't change, the water velocity is also increased. This can be further explained using the following formula:
Q = VA
Q = Flow rate, m3/s
v = Water velocity, m/s
A = Pipe cross-sectional area, m2
We know that the total pressure in a closed loop chilled water system is static pressure plus dynamic pressure, the higher the water velocity, the greater the dynamic pressure. Thus, total pressure does increases with the pump head.
Let's say we double the water velocity. So, the new dynamic pressure is:
Pd = 0.5ρv2
Pd = (0.5)(1000)(62)
Pd = 18000 Pa
Pd = 18 kPa
Pd = 0.18 bar or 2.61 psi
As you can see, increasing the water velocity will exponentially increase the dynamic pressure and thus, increasing the total pressure. However, the overall effect is still insignificant.
So, we may say that increasing the pump head does very little to the total pressure. Hence, the pressure rating of valves and fittings don't necessarily need to upgrade with the pump head.
If you don't mind going back to textbooks, all the above equations and calculations are mostly based on Bernoulli's principle. I find the below equation is particularly helpful:
P1 + ρgh1 + 0.5ρv12 = P2 + ρgh2 + 0.5ρv22
Messing around with Bernoulli's equation helps me to understand that the velocity of water changed only when the pipe size changed. Nothing to do with the pressure.
Besides, I also realized that if a pipe goes from a small size to a big size, water velocity drops and pressure increases. Water pressure increases when the velocity decreases.
In addition, if you don't have enough pump heads, the flow rate is going to be small and water velocity may be smaller depending on the pipe size.
Furthermore, the below diagram illustrates the static pressure and the dynamic pressure very well for a moving fluid:
As you can see, the water height (h1) in the pitot tube is higher than the water height (h2) in the piezometer. The pitot tube is submerged in the water while the piezometer is on the surface of the pipe.
The piezometer represents the static pressure while the pitot tube represents the total pressure. Because the pitot tube is submerged in the water and not moving with the water, it experiences an additional dynamic pressure. Hence, the water raises more in the pitot tube than the piezometer.
Pressure gauges installed on chilled water pipes are like the piezometer which may not give you the total pressure. On the other hand, the pitot tube is like valves on chilled water pipes which are constantly experiencing both static pressure and dynamic pressure.
All and all, I find that the weakest point (highest pressure point) in a closed loop chilled water system perhaps is the valves and fittings that have the highest pressure drop and are installed on the biggest pipe on the lowest floor.
Back to the starting question, why do so many HVAC engineers calculate the pressure of a closed loop chilled water system by adding the static pressure and the pump head?
From my understanding, it is a common practice and sort of like a safety factor to ensure pipes and valves do not burst. However, the safety factor sometimes is too much.
For example, it is very common for the chilled water pump to have a pump head of 20 meters or 2 bar. If a chilled water pipe has a height difference of 70 meters, the calculated pressure by the traditional way will be 7 bar plus 2 bar which results in 9 bar.
When the system pressure is 9 bar, you can't use PN10 valves and fittings. Instead, you need to use to next pressure rating which is PN16 for all valves, fittings and cooling coils.
Obviously, the higher the pressure rating, the higher the cost.
Instead, if HVAC engineers understand the total pressure in a closed loop chilled water system is in fact just the static pressure and a little bit of dynamic pressure, a huge amount of money can be saved by using a lower pressure rating for all valves, fittings and other components.
For more information about chilled water system, purchase and download the Chilled Water System (eBook).
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