A New Approach To Set Point Control In Chilled Water Loops

This paper presents a new approach to control variables in chilled water loops for energy conservation. Based on the analysis of the relationships between differential pressure and chilled water flow rate in chilled water loop, a series of optimal set points can be found to reset the controlled variables. With these reset set points, chilled water pumps only provide the necessary energy for water distribution loops. An optimization problem accompanied with its constraints is proposed for direct-return chilled water loops. A simple algorithm is provided to find the optimal set points, which can be used in real time to control the variable speed drive pumps to save energy under part-load conditions. With some modification, the proposed approach can be extended to different type of chilled water loops. An application example shows that the chilled water loop operating under the optimal set points has the potential energy savings compared with the conventional method. NOMENCLATURE a0,a1,a2: curve fitting constant of pump energy; c(HVL): constraint function of HVL; Cpw: specific heat of water under constant pressure; gc: a constant; HBL: head friction loss of piping branch; HCL: head friction loss of cooling coil; HP: distribution pump head; HPL: head friction loss of pipings; HVL: head friction loss of control valve; HVL,0: pump head ; K1: friction factor of cooling coil; K2: friction factor of pipes; m: the total number of subsystems; mw: water flow rate; n: the total number of coils; Ppump: power consumption of pump; Qcoil: cooling load of coil; TCHWR: chilled water return temperature; TCHWS: chilled water supply temperature; ηmotor: efficiency factor of motor; ηpump: efficiency factor of pump; ηVSD: efficiency factor of VSD; Subscript B: branch; i: the ith coil/terminal; j: the jth coil/terminal; i.j: the ith coil in the jth subsystem; m: the measured data; des: the designed data; INTRODUCTION The chilled water loop is one of important subsystems in a typical central cooling system. It consists of pumps, pipes, valves, and controls (ASHRAE 1999). Chilled water loops transfer the heat from the conditioned areas to central plants. The chilled water from the chillers in the central plant is supplied to coils and terminals of various zones at chilled water supply temperature and water returns at chilled water return temperature. The chilled water pumps provide the energy for water to circulate through the distribution loops. The water flow rate across each cooling coil is controlled by corresponding valve. Our objective is to provide every coils or terminals appropriate chilled water flow matched with their cooling loads by the least power consumption. Before Variable Speed Drive (VSD) and Direct Digital Control (DDC) techniques existed, the chilled water pump delivered the chilled water to distribution loops at the constant flow rate regardless of the variation of cooling loads. This method not only wasted the pump energy but also resulted in the low ∆T problems. When VSD technique tended to maturity, many experts considered it to improve the controls of chilled water loop. It was believed by Hegberg (1991) that VSD pumping gave systems a high degree of control and potential operating power savings. In Rishel’s paper (1991), he gave the objectives of the control of VSD pumps in chilled water systems. However, controlling the chilled water loop only by VSD technique is considered not enough by some other experts. Nowadays, VSD and DDC techniques are widely accepted to be the powerful tools for energy conservation to control chilled water loops in Heating, Ventilating and Air-Conditioning (HVAC) systems. However, some problems still exist in implementation of VSD and DDC techniques. Ahmod (1991) proposed a DDC-based system for a direct-return hydronic system by measuring and calculating water flow rate for each coil. However, it is too complicated for engineering and almost impossible to install flow sensors and differential pressure sensors in each coil in practical. Using valve position to control pump speed through DDC system was proposed by Tillack and Rishel (1998), but the authors only give the general idea and no practical methods for common systems. In 1998, Hartman pointed out that VSD and DDC technologies were overwhelmingly underutilized and fixed differential pressure control could not reduce the pumping energy significantly at part-load conditions. He claimed that DDC network could be employed to operate the pump speed, however he didn’t give any practical methods to implemented his ideas. By analysis the traditional system curves, fan laws and pump affinity laws, Bynum and Merwin (1999) concluded that the design system curves could not be used to calculate part-load energy savings with variable flow systems. He provided a control concept of differential pressure reset for a specific application example according to engineering experiences. Because the characteristics of chilled water loops are different from system to system, Harris’s control concept was hard to extend to the other systems. ASHRAE (1999) states that the best strategy for a given chilled water set point would be to reset the differential pressure set point in order to maintain all discharge air temperatures with at least one control valve in a fully-open condition. But how to find the fully open valves and implement it in real time with variation of cooling load is main difficulty. A new approach to set point control of VSD pump speed and control valve positions is proposed in this paper to deal with the difficulty. Based on analysis of the relationships between differential pressure and chilled water flow rate in chilled water loop, an optimization problem accompanied its constraints are given. Through a simple optimization algorithm, the optimal set points can be found out promptly and be implemented into control in real time for energy conservation. Compared with the traditional control method, potential energy savings can be achieved by the new approach. SYSTEM DESCRIPTIONS The typical variable flow chilled water distribution system is shown in Figure 1. Each distribution piping branch consists of a cooling coil and a two-way control valve. The two-way control valve is modulated by off coil air temperature (a room thermostat) in response to the varying room load. The VSD pump speed is modulated by a differential pressure controller usually located at the far end of the direct-return piping load as the system flow varies in order to keep a constant pressure differential set point. The differential set point is often chosen on the basis of the design condition and after being applied a safety factor. This kind of chilled water system was criticized by some experienced experts (Hartman 1998, Bynum and Merwin 1999) because it could not save the operating energy as expected in design phase. Fixed differential pressure control is the main reason of the inefficiency. Because the fixed differential pressure must be maintained, the head of VSD pump cannot be reduced as the pump law and the pump energy is wasted. In order to resolve this problem, the following system (Figure 2) by different control strategy is proposed to save the pumping energy under part-load conditions. In the new scheme, the VSD pump is no longer merely controlled by differential pressure sensors. It is controlled by optimal set points provided by a DDC controller, which collects the cooling load data of each coil. The control valves are no longer controlled by room temperature sensors. Instead, they are controlled by a DDC controller too. Figure 1. Typical chilled water distribution system Figure 2. Proposed chilled water distribution system The control strategy is given in following. • The DDC controller collects the chilled water supply temperature (TCHWS) and the chilled water return temperature of each distribution branch (TCHWR,i). • The DDC controller collects the differential pressure values of all the cooling coils (HCL,i) and determines the measured chilled water flow rate across the coils (mw,i,m) according to the coil characteristics, ( ) i CL m i w H f m , , , = . • Based on the measured data, the DDC controller calculates the cooling load of each coil ( ( ) CHWS i CHWR pw m i w i coil T T C m Q − = , , , , ) and the new required water flow rate ( ( ) CHWS des CHWR pw i coil i w T T C Q m − = , , , ) to achieve the designed chilled water return temperature (TCHWR,des) by the following two equations. • By optimization method introduced in the next section, the DDC controller determines the new set points of VSD pump speed and the control valve positions in every distribution branches. The VSD pump speed is determined by water flow rate ( ∑ = i i w w m m , ) and head of the pump (HVL,0). The control valve position is given in term of head loss across the valve (HVL,i). • The optimal set points are sent to local controllers or actuators to control the whole system. PROBLEM FORMULATION AND OPTIMIZATION The objective of optimizing the chilled water system is to deliver the enough chilled water to all the cooling coils at the least expenditure. The main energy consuming components are the distribution chilled water pumps equipped with VSDs. Therefore, the objective function is described in the following function according to mechanism model. C oi l C oi l C oi l DP DDC controller VSD