How to configure pumping schemes in HVAC applications
Join us as we guide you through various pumping schemes and show you how to get the most from your commercial building's HVAC application.
Welcome to the module on the configuration of pumping schemes. We're going to jump right into things and take a look at a constant primary flow pumping scheme.
As you can see in this scheme, chilled water is pumped at a constant flow rate independent of the cooling load. The three-way control valves at the cooling coils are used to bypass the chilled water back to the return line.
Since the unused chilled water diverted by the three-way valves mixes with the chilled water, the return chilled water temperature is reduced, lowering the plant's Delta T. Reduced Delta T has a significant influence on the installed chiller capacity of an HVAC plant. For instance, if the return water temperature received by the chiller is 12 degrees celcius instead of the normal 14 degrees celcius, the chiller capacity is reduced by almost 30%.
To overcome this deficiency, plant operators rely on extra chillers and pumps. This, however, consumes extra energy. Simply put, achieving the correct Delta T is essential for efficient plant operation.
In order to avoid this so-called Delta T syndrome, it is crucial that the pumping schemes in our air-conditioning systems are configured correctly.
In a typical primary-secondary system, chilled water flows through the chiller primary loop at a constant flow rate. In the secondary loop, the flow rate is varied by means of a control system. Thanks to the hydraulic independence of each loop, the variable flow in the secondary loop has no influence on the constant flow in the primary loop.
In a scheme like this, the primary pumps are constant speed pumps designed to deliver constant flow in the production loop. The secondary pumps are variable speed pumps, delivering a varied flow according the load requirement, ensuring that only the energy required to meet the demand is used, bringing significant energy savings.
When working with such a pumping scheme, adding a decoupler - also known as a neutral bridge - is crucial.
In a primary-secondary scheme, a decoupler divides the chilled water system into two distinct circuits that are hydraulically seperated. Without a decoupler, the primary pumps, chillers, secondary pumps and cooling coils would be connected in a series.
And when they're connected in a series and everything works in a loop, there is an increased risk of a breakdown due to a limited flow in the primary circuit when the secondary pump speed varies with respect to load changes. So, if there is no decoupler, there is risk of the airconditioning plant shutting the system off.
So, now we know why it's so important, let's take a look at what the decoupler does.
Simply put, the decoupler allows for the free flow of chilled water from the supply header to the return header and vice versa.
Mixing the chilled water balances the flow in the air-conditioning plant room whenever chillers are added or removed from operation by the plant manager or the building management system.
Furthermore, the direction of the chilled water flow in the decoupler can be used to control chillers as well.
When sizing a decoupler in a constant primary-variable secondary pumping scheme, there are a number of things you need to be aware of:
First of all, you need to make sure that the size of the decoupler is designed for the largest primary pump or largest chiller in the plant room. If the decoupler pipe is under- or oversized, it will create resistance against the free flow of chilled water, affecting the Delta T and the plant's overall performance.
Secondly, the distance between the T-joints located between the supply and return chilled water headers must be at least 3 times the diameter of the decoupler line to avoid an unwanted pressure drop.
Finally, lengthy decoupler lines will experience a higher pressure drop and, thus, they will not allow for proper mixing. On the other hand, decoupler lines of a shorter length will allow short circuiting of chilled water back to the plant, resulting in a low delta T. So, achieving the right length in your decoupler line is critical.
In the final part of this module, we're going to take a look at how a pumping scheme works with changing load conditions.
During the day, the cooling demand in a typical commercial building increases, which subsequently increases the air temperature of the conditioned space. To meet the increased cooling demand, the two-way valves open, causing a drop in the system differential pressure. This drop is subsequently detected by a differential pressure sensor, which adjusts the speed of the secondary pump to match the differential pressure setpoint in the system, compensating for the drop.
Likewise, when the heat load decreases at the end of the day, it triggers a drop in the air temperature, causing the two-way valves to close. This increases the differential pressure across the cooling loads, triggering the secondary chilled water pumps to slow down and thereby reducing the chilled water flow in the system.
That covers our module on pumping schemes.