ASHRAE Journal - July 2010 - 38

conventional method used to control the operating speed of pumps distributing water to terminal units. In this original control strategy, additional pump energy will be consumed at part-load conditions, especially at offdesign conditions. When the load of the terminal units reduces, the modulating valves need to close down to reduce the water flow rate at the primary side of heat exchangers. This causes the head of the pumps to be consumed by the modulating valves. Therefore, unnecessary pump power will be consumed.
Alternative Control Strategy

Temperature Setpoint Secondary Side of HX

Temperature Controller

Modulating Valves Primary Side of HX
M

From Terminal Units T To Terminal Units

HX From Cooling Source
M

To Cooling Source

Temperature Setpoint

Temperature Controller

Pressure Differential Setpoint

Differential Pressure Controller

Figure 2: Schematic of original control strategy.

To achieve energy-efficient control Secondary Side of HX Primary Side of HX of the pumps distributing water to heat exchangers, an alternative control From Terminal Units strategy (Figure 3) is proposed. In this HX strategy, the water flow measurements From Cooling To Terminal at primary and secondary sides of the Source M Units M T heat exchangers, and measurements of To Cooling the water temperatures at the secondary Source HX side of heat exchangers are used. The supply temperature at the secondary side is controlled by adjusting the water flow Temperature Water Flow Temperature Water Flow rate (pump speed) at the primary side Setpoint Controller Controller Setpoint while the original modulating valves are set fully open. A cascade control scheme Figure 3: Schematic of alternative control strategy. is used to provide stability and ensure that the difference between the water flow rates on both sides is controlled within an acceptable range. M up = min ((1 + ) M sec , M max ) The PI temperature controller (Figure 3) generates the water (2) flow setpoint of the primary loop. A simple linear relationship with the PI output is adopted to determine the water flow setM low = max ((1 − ) M sec , M min ) point (Mset) (Figure 4). Mlow and Mup are the lower and upper (3) limits of the water flow rates at primary side and are set as the parameters. The water flow setpoint is set within a range near the Compared with the original control strategy, there is no need actual water flow at the secondary side (many experts have sug- to use the modulating valves at the primary side when using gested that the water flow rates at both sides must be balanced). this alternative control strategy. Their selection is described in Equations 1 through 3. Msec is the measured water flow rate at the secondary side. Mmax and Mmin Performance Tests and Evaluation are the maximum and minimum limits of primary flow rate, Both control strategies have been implemented in the BAS which are set according to the water loop design parameters. of the building. Zones 1 and 2 have been occupied for more In this building, the value of α selected is 0.25. Mmax and than one year, while the other upper zones are still under Mmin are the design flow rate and 20% of the design water construction. Upon initial operation, the original control flow rate at the primary side. The value of 0.25 was selected strategy was used to provide the control function for the pumps based on site experiences and discussion with system designer in the primary side of the heat exchangers. and operators, ensuring that the difference between flow rates The alternative control strategy replaced the original strategy at both sides is not large. The value of 20% was selected to last summer. Since both low zones were not fully occupied, the ensure that the system operation can cover the possible allowed cooling loads in the zones were still very small, and the pump operation range. constantly operated near the lowest allowable operating frequency when both strategies were used. Therefore, the performances M low ≤ M set ≤ M up (1) of both strategies only can be tested and compared on site with
38 ASHRAE Journal ashrae.org July 2010

ASHRAE Journal - July 2010

Table of Contents for the Digital Edition of ASHRAE Journal - July 2010