Purpose of Today s Presentation

Transcription

1 Chilled Water Distribution Systems APPA Institute for Facilities Management Dallas, TX January 19, Purpose of Today s Presentation To provide a broad understanding of chilled water distribution systems Explore in some detail various distribution system configurations Provide some useful observations and solutions 2 Agenda System Concepts Definitions Basic Formulae T Hydraulic Profile System Components System Configurations 3 APPA Institute - Dallas, TX Feb

2 WORDS OF WISDOM It s not how much you ve got; it s whether you can use it. Production Distribution Load 4 Definitions System (Static/Fill) Pressure: The non-flowing pressure to which the system must be filled to assure flooding of the highest device. Static pressure is created by the weight of water in the system. Static pressure has no effect on pump capacity. If you consider a water piping system as being an upright loop of water confined in a pipe, the static pressure in one of the vertical pipes is caused by the weight of the water column in the pipe. Static Pressure is equal to.434 pounds per sq. inch per foot of water above the measurement gauge. For example, if the highest device is 20 feet above the gauge, the static pressure at the gauge will be: 20 x.434 which equals 8.6 psig. At various elevations above the gauge, the static pressure becomes correspondingly less. At 10 feet, it is 4.3 pounds per sq. in., and at the top, located 20 feet above the gauge, there is no pressure. System pressure is usually set so that there is at least 5 psig measured at the highest device in the system. QUESTION: What pressure must there be in the system if the highest device is located 120 feet above the chilled water makeup water inlet? 5 Fill Pressure, Makeup, and Expansion ΔH= 120 ΔH= ΔH120 Makeup/Fill Water Makeup/Fill Water System Pressure =.434 psi/ft X = 57 psig 6 APPA Institute - Dallas, TX Feb

3 Definitions (cont.) Dynamic Pressure: The flowing pressure the system pumps must develop to overcome the friction due to piping, coils, valves, fittings, and other devices in the system at a given flow rate. Head loss, measured in feet of head = 2.31 ft. W.C./psi (1/.434 psi/ft) Design Pressure The dynamic pressure the system pumps must develop at the maximum flow in the system. The differential pressure between the supply and return piping at the pump, i.e. the total head QUESTION: What will the supply and return pressures be in our 57 psig system if the design head loss at maximum flow is 100 W.C.? Supply Pressure = 100 W.C. X.434 psig/ft + 57 psig = 100 psig Return Pressure = 57 psig 7 System Hydraulic Profile 100 psig Pressure 57 psig Total Head = 100 Plant s Typical Bldg Load Supply Piping Return Piping Relative Distance from Plant 8 Basic Formulae The heating and cooling capacity of water when it flows through a coil (heating or cooling) can be calculated as follows: Basic equation: Q = mc p ΔT = ρc p VΔT for water: Q = 60min/hr V 8.33 lb/gal 1.0 BTU/lb- o F T = 500 x GPM x T Q = heat rate (Btu/hr, kj/hr) Converting to refrigeration tons: m = mass flow (lb m/ /hr, kg/hr) c Q Tons = 500 x GPM x T p = specific const. press. ρ = density (lb/cu. ft.) 12,000 BTU/Ton-hr ΔT = temperature difference between supply and return GPM T Q tons = 9 APPA Institute - Dallas, TX Feb

4 Chilled Water System Component Interactions s/ Piping Parallel ing Series ing Variable Speed ing Effect of T on Energy Effect of T on Flow Effect of T on Dynamic Pressure 10 ing Arrangements 2 s 2 s Q tons GPM T = Varying Speed 12 APPA Institute - Dallas, TX Feb

5 Delta T vs. Req d System HP Q tons GPM T = ΔT vs. System HP For Fixed Load Horsepower Temperature Difference 1.8 HP 13 Specific Flow vs. ΔT System HP ~ Q 3 14 Dynamic Pressure vs T GPM T Q tons = Increasing supply-to-return differential temperature requires less flow for same heat transferred Less flow in a given pipe system results in lower velocity Lower velocity equals lower friction and lower pressure loss Lower pressure and flow equals lower energy Three Rules for Chilled Water System Optimization Reduce Flow Reduce Flow Reduce Flow 15 APPA Institute - Dallas, TX Feb

6 Chilled Water Distribution System Configurations Constant/Variable Flow Combinations Primary Primary/Secondary Primary/Secondary/Tertiary Variable Direct Primary 16 Constant Primary Only (One unit on) Control Valve Load equals 1 chiller 12 o F T = 17 CV Constant Primary Only (Two units on) Control Valves bypass excess water into return Load equals 1.2 chillers = 600 Tons = o F T 18 APPA Institute - Dallas, TX Feb

7 Constant Primary / Secondary Building Secondary s Bridge 19 Constant Primary / Secondary / Tertiary Secondary Building Secondary s Bridge 20 Constant Primary / Variable Secondary (primary and secondary pumps in central plant ) Variable Secondary 3000 gpm max. Control Valve Bypass (Bridge) System flow more less than chiller flow staging indicated by flow direction in the bridge 21 APPA Institute - Dallas, TX Feb

8 Variable Primary Only (One unit on) VF Control Valve Load equals 1 chiller 12 o F T = 22 Variable Primary Only (Two units on) QUESTION: How can we improve this scheme? VF 600 gpm Control Valves close against increased pressure VF 600 gpm and flow staging accomplished by measurement of P between supply and return at selected location Load equals 1.2 chillers = 600 Tons = o F T 23 Questions & Answers Thank You! APPA Institute - Dallas, TX Feb