Monday, March 11, 2013

Sizing A Circulator Just Takes Some Math

The dictionary defines a system as a group of interacting, interrelated, or interdependent elements forming a complex whole. When designing and installing a hydronic heating system, we are in fact creating a group of elements that interact, that are interrelated and are interdependent, and they do form a very complex whole that is designed to keep people comfortable while minimizing energy usage. The system’s relative success or failure depends on how well these elements work together to perform the system’s stated function. The definition of hydronics is the science of transferring a definitive amount of BTUs from a source device to a heat transfer device and back via the movement of water or solution thereof. A key component of a modern hydronic system is the circulator and its main function is to move heated water (BTU/HR) through a distribution system (the radiators) and back.

It’s important to remember than when sizing a circulator, you do not need to take into account the height of the building.  The physical height of the building does NOT equal the feet of head. As part of defining a circulator as opposed to a pump is the fact that we are in a closed loop system versus an open system which has to over come static head as well as pressure drop. Examples of this would be a well or a sump pump system. The circulator does not need to lift the water to the top of the building due to the simple fact that what goes up must come down.  The circulator doesn’t have to lift the water to the upper floors – the weight of the water coming back down the return side is a counterbalance. Think of the circulator as the motor on a Ferris wheel.  The motor doesn’t have to lift the weight of the people up – there are people on the other side of the wheel coming back down. All it has to do is overcome the friction loss of the bearing assemblies in the wheel.  A circulator doesn’t have to lift the water – it only has to overcome the friction loss – or head loss – of the system. 

All piping systems impart friction loss on the fluid in the system, and understanding this is key to making sure your hydronic system functions properly. If you do the math, calculating the flow requirements for circulator is pretty simple - it's basic arithmetic. Calculating the "other" half - head pressure (or friction loss) - is a little tougher. Use the Universal Hydronics formula to determine how much flow the circulator has to be capable of.

GPM = BTUH ÷ ΔT x 500

GPM is gallons per minute. BTUH is the calculated system load. ΔT is the temperature difference across the system at design conditions and we use 20° F for our systems. 500 is a constant - it is the weight of a gallon of water (8.33 lbs.) times 60 minutes. When we have determined the load of the system all we need to do then is to divide by 10,000 (20 x 500) and we have our GPM requirement for the circulator. As an example, let’s say we are zoning with circulators and have a 30,000 BTU zone of baseboard or 50 feet of element. When we divide the 30,000 by 10,000 we determine a flow rate of 3 gallons per minute.

Choosing The Pipe Size
What size pipe should we use for this zone? Well, the guidelines for pipe sizing are as follows: 
  • 2 to 4 gallons per minute of flow, use ¾” M copper;
  • 4to 8 GPM, use 1 inch;
  • 8 to 14 GPM, use inch and a quarter;
  • 14 to 22 GPM, use inch and a half. 

These all fall within hydronics guidelines for pipe sizing and keeping flow velocities at no less than 2 feet per second and no more than 4 feet per second. At velocities greater than 4 feet per second, the system will produce velocity noise and customer complaints. At velocities lower than 2 feet per second, dissolved oxygen will tend to come out of solution and cause air problems within the system. 

To determine the head loss of a zone, start by measuring the total length of the zone, including the element. In this case we have 80 feet of  ¾” pipe connected to the 50 feet of element for a total of 130 feet.  Now multiply the total by 1.5 to allow for fittings, valves, etc. Fittings and valves produce pressure drop in a system that is the equivalent of a few feet of pipe each, so multiplying by 1.5 accounts for most basic fittings and valves.

If you have high head items in your system like flow checks or 3 way valves, you will have to add some more head later. You now have the total developed equivalent length of the circuit and you multiply that by .04. This number represents 4 feet of head per 100 feet of copper pipe. That head number applies as long as the pipe has been sized according to the velocity guidelines shown in the previous paragraph.  The end product is the head loss for the zone.  120 x 1.5 x .04 = 7.2 feet of head. We must now find a circulator that will deliver 3 gallons per minute against 7.2 feet of head.

If we take a look at the Taco “00” series performance curve chart – we can determine which circulator we should use for this zone. As long as the point at which the system operates is inside of or on the line that the pump operates, you are assured that the pump will deliver heat at the right temperature to the zone. If that point falls outside the pump curve, your pump will not be able to deliver the maximum amount of BTUs needed under design conditions. Simply put, in the coldest weather, the system can’t reach the required comfort level. In the case of an indirect water heater it will be slow in recovery.

First, on the bottom axis, we find the flow rate – in this example, it is 3 gallons per minute. On the vertical axis we have head loss – in this example it is 7.2 feet of head. We follow the two lines until they intersect to find our system operating point of 3 GPM at 7.2 feet of head. Next, we look at the performance curves to find out which circulator would make the best selection.  In this example, a 006, 005 or a 007 would be good choices – with the most likely choice being the 007, since it’s the most common and most readily available. 

Once you understand the dynamics that are going on in a system, you can size and select the right circulator for the job. Learn more about Taco products on our website