HVAC systems normally have large range of load demands
by Ray Hardee
December 11, 2017

In last month’s column (read it here), we looked at cooling water service systems with multiple hydraulic circuits used to cool equipment in industrial/process industries. These systems typically have a constant flow rate because most of the equipment being cooled is on when the facility is in operation.

Heating, ventilation and air conditioning (HVAC) cooling water systems have multiple hydraulic circuits to provide chilled water to an air handler consisting of a water-to-air heat exchanger and an actuated temperature control valve to regulate the flow rate through the chilled water circuit.

As previously discussed, when a single pumping source supplies multiple circuits, the flow rate through one circuit affects the flow rate through
all circuits.

Since the flow rate through an HVAC chilled-water system varies based on the system heat load, and that heat load is affected by the time of day and the occupancy of the conditioned space, HVAC systems typically have a large range of load demands. These changing load demands, along with the large number of circuits within a typical HVAC system, create difficulty in maintaining proper system control.

A Little Bit of History

Previously, HVAC systems used a constant volume method of control (see Figure 1). The flow rate through each circuit was balanced to maintain a constant flow rate regardless of the heat load of the air handler. This was accomplished by installing balancing valves within each circuit upstream of the control valve to limit the flow rate through the circuit to the maximum flow rate. To regulate the outlet temperature of the air handler, a three-way throttle valve was used.

control volume system graphicFigure 1. A simplified example of a constant-volume cooling water system in which the design flow goes to each circuit regardless of the head load (Graphic courtesy of the author)

The design flow rate of the circuit would go into the three-way valve inlet. Based on the position of the valve stem, a percentage of the total flow would go to the air handler to maintain the outlet temperature of the air handler. The remaining design flow for the circuit was diverted through the three-way valve to bypass the air handler.

This approach provides a constant flow rate through the chiller water system once the system was balanced, and the use of the three-way valves maintained a constant outlet air temperature of the air handler. The primary disadvantage of this approach required a constant flow rate through the system regardless of the HVAC load.

Within the HVAC industry, unique ways were tried to optimize the system. Prior to the 1980s, constant volume systems were the standard.

Employing Pumps with VSDs

With the availability of low cost variable speed drives (VSDs), many pump manufacturers offered their pumps with VSDs. The VSD allowed the speed of the pump to change by reducing the differential pressure across the control valves. This method of control worked well for industrial and process applications, but there were many problems applying VSDs to the HVAC market. The first time I saw an HVAC chilled water system with a VSD was on a constant volume system. The speed control was used specifically for system balancing.

By adjusting the speed of the VSD, the balancing contractor could easily assess the pump head to minimizing the head loss across the balancing valves. This did save some money on pumping costs, but the potential benefits of using a VSD could be much greater.

Challenges in HVAC Chilled Water Systems

Due to the size and complexity of HVAC chilled water systems, it was difficult to arrive at a standardized method for utilizing the full potential VSDs have to offer. Every piping system is made of pump elements, process elements and control elements. It is impossible to understand how a system operates without considering how the three elements work together.

The first approach after installing a VSD on a pump was to replace the constant volume system with a variable volume system. The three-way control valves were replaced with standard two-way control valves. The standard valve would regulate the chilled water flow rate through the circuit to control the outlet air temperature of the air handler. By adjusting the pump speed with the VSD, the excessive differential pressure could be reduced across the temperature control valve, therefore achieving greater savings in energy cost.

That approach worked best on new chilled water systems, but there were problems when applying it to systems that had been added to over time, or to systems with wide variations in HVAC cooling loads. Many of the manufacturers providing controls in the HVAC space developed differential pressure control valves and pressure independent control valves to help address the problem of high differential pressure across the temperature control valves. Pump manufacturers would also suggest booster pumps to solve some of the problems associated with providing sufficient flow to all circuits.

Many of these products worked for various types of systems, but most often the addition of new equipment typically resulted in haphazard improvements.

System Knowledge Provides the Way

These HVAC systems can be rather large, are often added to over time and have a wide variation in system loads. As a result, it is difficult to understand how the various elements making up the chilled water system work together.

Over time, many of the owners and operators of these commercial buildings realize that something must be done to correct the problems with their air conditioning system to keep their customers satisfied.