Circulation Pumps for Closed Loop Systems
One of the most commonly misunderstood concepts when it comes to the mechanical systems of buildings is the circulation of a closed loop system. The concepts of Net Positive Suction Head (NPSH), static head pressure, pressure drop, and others are often misapplied when it comes to a closed loop circulation system. In this article, we will give some guidance on the proper application of these theories and the terminology to be used when discussing the circulation of a closed loop system.
First, we should define what we mean when talking about a closed loop system. A closed loop system is any circulation loop where a particular medium (water, glycol, etc.) is continuously circulated around a system of piping. The medium is typically used for heating and is being heated and cooled via mechanical equipment (i.e. boilers, heating tanks, radiators). In these systems, a feed valve is used to fill and pressurize the loop and replace any water lost due to faulty gaskets, loose fittings, or other unintentional leaks.
The feed valve, which is just a pressure reducing valve, is initially set so that incoming water can overcome the static head pressure in the loop. For example, as water fills a system that covers a five-story building through a feed valve located on the first floor, the water pressure seen on the first floor will increase as the water level rises in the system. When this system is full, there will be approximately 45’ of vertical distance between the water on the 5th floor and the water on the 1st floor. If the water on the 5th floor is at 0 psig, a pressure gauge on the 1st floor would still read about 19.5 psig due to the weight of the water pressing down (static head pressure). The set pressure of the feed valve would have to be at least 25 psig to overcome the static head pressure and ensure that there is still a level of pressurization at the highest point of the loop.
While most readers would understand the concepts mentioned up to this point, the next component is where a lot of misunderstanding takes place. Now that the closed loop is full of water there must be a means to circulate the water around the loop; the circulation pump. The first parameter used to select the appropriate pump is the flow rate which is determined by a number of other factors and will not be covered in this article. The next important pump characteristic is the discharge head or the pressure increase that the pump is able to produce. There is a common misconception that the discharge head is required to pump the water up to the tallest point of the loop but that is the job of the water pressure from the feed valve. The loop is already full and now needs to be circulated around. The discharge head of the pump must be equal to or greater than the total pressure loss due to the water flowing around the loop. The pump is selected to overcome these losses at the flow rate required.
In a closed loop system, the pump discharge head is never selected on the basis of the building height but on the total system pressure loss. However, the Net Positive Suction Head Required (NPSHR) is also an important pump characteristic to keep in mind when selecting and placing the circulation pump. To avoid cavitation at the impeller of the pump there must be sufficient pressure at the supply connection. This is the reason why circulation pumps are typically on a lower floor with the other mechanical equipment.
While this article certainly does not cover all of the information required to design a closed loop system, the intention was only to provide some initial clarity on the circulation pump of a closed loop system.