What is a Series-Counterflow HVAC Chiller Plant?

Series-counterflow has been around for several decades but only really utilized for the past decade due to advancements in chiller technology. If you’re looking for LEED credit for your building or rebates for using more efficient equipment, this could be a route to take.

The concept revolves around reducing the work that a water-cooled chiller must do by decreasing the lift the machine has to overcome within the refrigerant cycle. To identify lift, you subtract the existing condenser water temperature from the leaving evaporator water temperature. Typically, in South Carolina, North Carolina, Virginia, Georgia, and Tennessee, we see 45 ℉ chill water temperature going to the comfort cooling air handlers (or to some process heat exchangers) and 55℉ returning from the space (or process) to the chiller. On the condenser waterside, we see 85℉ entering the chiller from the cooling tower or spray pond (you rarely see spray ponds now, a legacy from older industrial plants) and 95℉ leaving to be cooled down again. To find the lift, you would simply subtract:

95℉ – 45℉ = 50℉ of lift

Series counterflow takes that same 55℉ temperature water returning from the building or process heat exchanger and lowers it to only 50℉. The water then goes into a second chiller that cools it to 45℉. Now that second chiller with the 45℉ exiting to the space has condenser water from the cooling tower entering in at 85℉ and rising to 90℉. That 90℉ water then enters the first chiller that has 55℉ water from the space raises the condenser water from 90℉ to 95℉.

Chiller 1: 95℉ – 50℉ = 45℉ of lift

Chiller 2: 90℉ – 45℉ = 45℉ of lift

Both machines are now doing 5 degrees less lift or less work, making them more efficient. Simply put, we have the chillers in series, and the evaporator water is running in counterflow to the condenser water, hence the term series-counterflow.

What would a typical solution for series-counterflow look like?

I’ll give you two real-life examples. One was out of necessity, the other for LEED credit.

Example 1: Necessity

A real-life design-build project I helped design and install was for an industrial site with mill water with summertime temperatures of 90℉ that needed to cool to 45℉ to be used in their process. The water that went through the machines never came back as a loop. All chilled mill water was utilized in the making of the product. The water was pulled from a nearby river and held in concrete retention ponds with sand filters before entering the site.

Since the plant started up, they would have summer days that they could not make product. They asked me to provide a solution that would allow them to make product regardless of ambient conditions. The potential upside to having this ability would be seven figures in profit per year for the client. A standard water-cooled chiller can’t overcome such a high delta in temperature because there is only so much heat energy that can exchange via refrigerant and a heat exchanger in a standard machine. The lift or work done by a standard machine was too great in this scenario. The owner wanted to stick with a standard machine because of the unique problems specialty machines face when needing repair: Problem one is that only so many mechanics know how to work on one-off machines. Problem two is that if a part fails, they are usually not readily available for a specialty chiller. Lastly, you are heavily reliant on the specialty chiller manufacturer to stay in business and keep support staff to troubleshoot the model you purchased.

Our solution was to provide two centrifugal chillers that were run in series-counterflow. Chiller 1 would receive 90℉ mill water and lower the temperature to 67.5℉ on the evaporator side. Chiller 2 would pick up that 67.5℉ water and reduce it to 45℉ to serve their process. On the condenser side, Chiller 2 would pick up 90℉ mill water and increase the temperature to 100℉. Chiller 1 would also pick up 100℉ water and reject it to their hot water line at 110℉.

Chiller 1: 110℉ – 67.5 ℉ = 52.5℉ of lift*

Chiller 2: 100℉ – 45 ℉ = 55℉ of lift

*More robust impeller and multi-pass shell and tube heat exchangers were used to overcome the higher-than-normal lift.

As a part of this design, we made sure to have the machines robust enough to provide some cooling if one of the chillers had to come down for maintenance or repair. To give the client peace of mind, we offered a service agreement to keep the maintenance on both chillers up-to-date and make any repairs that we found during their plant-wide annual outage.

Example 2: Energy Savings

The second real-life solution was for energy rebates from the power company for a building that was getting upfitted.

The building had a 700-ton load historically served by two 350-ton machines running in parallel running at a 10℉ ΔT. We modified the piping and set them in series-counterflow. We also increased the ΔT from 10 degrees to 15 degrees. Why? By doing this, we were able to put in smaller air handlers selected for a wider delta. We decreased the lift or the amount of work the chillers did while improving the intermittent part-load value (IPLV).

The energy savings came to over $150,000 per year by making this simple modification. Their rate with the power company was six cents per kilowatt-hour. Yes, our power is cheap in the southeast.

To conclude, if you are interested in taking advantage of these benefits by modifying your chiller plant to accommodate series-counterflow, our team at HMS is ready to work with you. If you have a water-cooled or air-cooled chiller plant and would like some outside-of-the-box ideas other than series-counterflow, don’t hesitate to contact us at service@hoffmech.com or 1-855-761-HVAC.