How to Size Condenser Water Pipes

The pipe that connect from the condenser side of the chiller to the cooling tower and back is called the condenser water pipe. To size a condenser water pipe, we can use a friction loss chart.

There are two types of friction loss chart: a) closed piping systems and b) open piping systems. Condenser water pipes are in the open loop circuit of the chiller system as opposed to chilled water pipes which are in the closed loop circuit. Therefore, we need to use the friction loss chart for open piping systems to size condenser water pipes.

I attached herewith the friction loss chart for opening piping systems by Carrier. You can download it here for free. In the following, I’ll explain how to use the chart to size condenser water pipes in different piping sections.

Condenser Water Pipe Sizing

To size a condenser water pipe, we first need to determine how much condenser water flow in each pipe section. For example, if you have a 750-ton chiller, using a rule of thumb of 3.0 gpm per ton, the required condenser water flow rate is 2250 gpm.

Now, we can use the friction loss chart to size the condenser water pipe that’s going to be connected to the condenser side of the chiller.

Step 1 – Set the Velocity Limit

Mark the velocity at 6 fps and 12 fps. That’s the range we want the condenser water to flow in and out of the chiller.

For this pipe section, we are designing at a slightly higher velocity (normally is 5-12 fps) to reduce fouling which is the accumulation of dirt inside the condenser tube.

However, we don’t want the velocity to be too high either. Many chiller manufacturers recommend don’t go over 12 fps due to high erosion rate.

Step 2 – Mark the Flow Rate

Draw the flow at 2250 gpm horizontally across the chart.

Notice that I circle all the intercept points between our green line (flow) and the pipe size. The pipe size is the line that goes up from left to right.

Now, there are 8 intercept points which mean that we can choose 8 different pipe sizes for 2250 gpm.

However, we want to narrow it down and consider only those that are within the region we just marked because we want to let the condenser water flow in and out of the chiller at between 6 fps and 12 fps.

Step 3 – Select the Pipe Size

So, only 10-inch and 12-inch pipe fall within our design requirement.

If we use 10-inch pipe, the velocity will be about 9 fps. If we use 12-inch pipe, the velocity will be a little over 6 fps.

Both pipe sizes are viable. 10-inch pipe will be cheaper but the friction loss is higher. 12-inch pipe will be more expensive but the friction loss is lower.

The respective friction loss can be checked by looking at the x-axis. For instance, the 10-inch pipe point is about 4 ft/100 ft.

I’ll briefly discuss about friction loss vs pipe cost.

Friction loss is amount of resistance inside the pipe. Obviously, the smaller the pipe, the higher the resistance.

On the other hand, the size and power consumption of a pump is affected by the total resistance or total friction loss in the piping system. If many of the pipes have very high friction/resistance, the pump will consume more power to deliver the required flow. Therefore, the operating cost is higher.

So, when sizing a pipe, it’s important to compare the initial piping cost and the operating cost. Sometimes, the initial piping cost is a critical surviving factor. Therefore, higher velocity and higher friction loss are often preferred over energy conservation and system lifespan.

Recommended Water Velocity

Carrier has outlined their recommended water velocity in Carrier Piping Design Manual for various services and the maximum velocity to minimize erosion based on annual operating hour.

As mentioned earlier, the condenser water pipe going in and out of the chiller should be flowing at between 6 fps and 12 fps. Other than that is as follows:

Annual operating hour means how many hours in a year do you expect the chiller system to operate. For example, the working hour in a typical office building is 9:00AM to 6:00PM, Monday to Friday. That’s 9 hours per day. Consider overtime, common areas operate outside the working hour and a few weekend operations, we may assume 12-14 hours per day, 5 days per week, 52 weeks per year, and the estimated annual operating hour is 3120 to 3640 hours per year.

So, we are allowed to a higher water velocity when designing the piping system.

However, certain sections of the pipe should still based on the performance like the chiller inlet/outlet. Normally, we can use the annual operating hour to design the supply/return pipe.

Minimum Water Velocity

While high water velocity lead to shorter lifespan, too low of a water velocity is not desirable either. Generally, most guidelines suggest a lower limit of 4 fps. On the other hand, ASHRAE recommends the minimum velocity is 2 fps, especially in higher floors.

I quote the following:

Anyway, keeping our sizing criteria within 5-12 fps should be fine for most applications.

Variable Flow

The sizing criteria is a bit different if our pumps are variable flow, meaning we use VFD/VSD to control the pump speed. Although condenser water rarely use variable flow, it’s good to know how it affects the sizing.

Variable flow means the amount of condenser water flowing through the chiller will change depending on various factors like cooling demand, outdoor conditions and etc. Otherwise is constant flow.

Earlier, we size our condenser water pipe based on 2250 gpm. We assume it is constant flow as it mostly be. Now, if it was variable flow, we need to use a smaller pipe size in order to ensure the velocity is still high enough when the condenser water flow rate is reduced.

For example, if the condenser water flow is reduced by 20% from 2250 gpm to 1800 gpm, the 12-inch pipe may not be the most ideal as the velocity drops below 6 fps. However, the 10-inch pipe becomes ideal as it still stay within the design criteria.

So, if variable flow is involved, we should analyze the cooling load profile to estimate the water flow range and then size the pipes and try to make them stay within the velocity range under all flow conditions.

A great way to do so is to learn how to calculate the 24-hour cooling load of a building. The ASHRAE’s Radiant Time Series (RTS) method allows you to do that and I happen to have an online course to teach you how to do perform the calculation with an Excel calculator. See the course outline here: RTS Cooling Load Calculation Course.

ASHRAE 90.1 Standard

As far as energy conservation is concerned, we have the ASHRAE 90.1 standard to follow when sizing condenser water pipes and any other relevant pipes.

ASHRAE 90.1 stated the maximum flow rate for 2-1/2 inch pipe and above. The limits are separated into three annual operating hour categories:

• 2000 hours/year and less
• More than 2000 but less than 4400 hours/year
• More than 4400 hours/year

For each category, ASHRAE further split the requirement into two types: a) variable flow and b) everything else (including constant flow).

Here’s part of the requirement for reference purposes:

However, the limitations set by the standard only applicable to pipes that are either in the critical circuit or have more than 30% of their operating hours in critical circuit. Critical circuit usually means the longest piping path which the pump head calculation is based on.

So, although the ASHRAE 90.1 standard is strict on the pipe size, it’s only account for a certain portion of the piping system. But, since condenser water pipes don’t really have many branches like chilled water pipes, the effect is much more significant.

Nonetheless, it doesn’t mean we can now size the non-critical circuit with much greater flow because it may become the critical circuit if the total friction loss increased significantly, although it is not the longest piping path.

I also highlighted the ASHRAE 90.1 requirement on pipe sizing on the friction loss chart. If you haven’t check it out, you can download it here. Thank you.