Definition: Balanced System

Transcription

1 Definition: Balanced System A BALANCED SYSTEM A balanced system in water side terms can be defined as one in which the terminal unit flow rates are adequate, under design circumstances, to maintain satisfactory heat transfer capability. Source: ASHRAE Systems Handbook

2 Definition: Manual Balancing A manual balance valve creates a fixed resistance in a circuit to insure design flow to all heat transfer devices in the system under full load conditions, usually by throttling flow. Fluid takes the path of least resistance so we have to force water out to the end of the piping circuit, and select a pump to overcome that head loss.

3 MANUAL BALANCING Circuit Setter Type (Manual)

4 MANUAL BALANCING Venturi Type (Manual) (Ball Valve for Throttling Purposes) Manually Adjusted by TAB Contractor Pressure and Temperature Ports Integral Ball Valve Type

5 MANUAL BALANCING Spring for mechanical play tightening Handwheel with digital indication (80 setting positions) Draining kit can be installed during operation 5

6 Definition: Automatic Flow Control AUTOMATIC FLOW CONTROL: A balancing device that prevents the flow rate from exceeding design flow, while absorbing dynamic pressure changes in the system. Real function is to act as a FLOW LIMITING DEVICE

7 Automatic Balancing CARTRIDGE / PISTON Various sizes available from 1/2 3 Accuracy ± 5 percent

8 Limitations In a Variable Flow System with Modulating Control. Neither Balancing Valve Allows us To:

9 Definition: Balanced System A BALANCED SYSTEM A balanced system in water side terms can be defined as one in which the terminal units receive the required flow rates under all operating conditions to maintain maximum heat transfer capability.

10 Definition: Balanced System A BALANCED SYSTEM A balanced system in water side terms can be defined as one in which the terminal unit flow rates are adequate, under design circumstances, to maintain satisfactory heat transfer capability. Source: ASHRAE Systems Handbook

11 Balancing for Variable Flow Systems Variable Speed Drive Sensor Placement Valve Authority Balancing

12 PUMPING COSTS VARIABLE SPEED PUMPS REDUCE PUMPING COSTS BY AVOIDING AN INCREASE OF THE PUMP HEAD WHEN THE FLOW DIMINISHES. Cost ~ Pump Head x Flow Pump head Pump head Savings Flow Flow

13 VARIABLE SPEED PUMPS H H Flow Flow H Flow

14 DIRECT MEASURE OF THE PUMP HEAD? ΔP A ΔP sensor is used to measure the pump head. At the location of the ΔP sensor, the available pressure will be kept constant. Where is the best location?

15 DP SENSOR AT THE END OF THE CIRCUIT ΔP Maximum pump energy saving When control valves shut, there is not enough available pressure for the first circuits. ΔP Is this the best location?

16 DP SENSOR AT THE MIDDLE OF THE CIRCUIT ΔP Improved situation The first unit does not receive enough pressure at low flow (when control valves shut). ΔP Is this a problem?

17 DP SENSOR AT THE BEGINNING OF THE CIRCUIT ΔP Best mode from operating condition point of view? Available pressure is always high enough for all circuits and for all operating conditions. ΔP

18 BASIC SYSTEM & DESIGN ASSUMPTIONS Control valve Delta P equal to coil Delta P Head Loss through each coil is identical for each circuit at 4 psi Balance Valve at index circuit has 2 psi Maximum Delta P of pump 40 psi DP Transmitter on index circuit 10 psi Each Circuit Has Design Flow of 100 gpm

19 APPLICATION BASIC SYSTEM DP Pressure Differential Sensor Circuit 4 10 psi Coil # gpm Δp=4 psi 10 psi Circuit 3 20 psi Coil #3 Circuit 2 30 psi Coil #2 30 psi Circuit 1 40 psi Coil #1 40 psi VFD Pump Chiller

20 CALCULATE BALANCE VALVE PRESSURE DROP TO OBTAIN DESIRED DESIGN CONDITION Circuit 1 = (40-4-4) = 32 psid Circuit 2 = (30-4-4) = 22 psid Circuit 3 = (20-4-4) = 18 psid Circuit 4 = (10-4-4) = 2 psid

21 APPLICATION BASIC SYSTEM DP Pressure Differential Sensor Circuit 4 10 psi 2 psi Coil # gpm Δp=4 psi 10 psi Circuit 3 20 psi 12 psi Coil #3 Circuit 2 30 psi 22 psi Coil #2 30 psi Circuit 1 40 psi 32 psi Coil #1 40 psi VFD Pump Chiller

22 CALCULATE CIRCUIT Cv Cv = GPM/ P Circuit gpm@ 40 psid = 15.9 Circuit gpm@ 30 psid = 18.2 Circuit gpm@ 20 psid = 22.2 Circuit gpm@ 10 psid = 31.3

23 APPLICATION BASIC SYSTEM Circuit 4 10 psi DP Pressure Differential Sensor Δp=2 psi Coil # gpm Δp=4 psi 10 psi Cv=31.3 Circuit 3 20 psi Δp=18 psi Coil #3 20 psi Cv=22.2 Circuit 2 30 psi Δp=22 psi Coil #2 30 psi Cv=18.2 Circuit 1 40 psi Δp=32 psi Coil #1 40 psi Cv=15.9 VFD Pump Chiller

24 DESIGN PROBLEM DESIGN PROBLEM It is possible to STARVE the coil closest to the pump under partial load conditions

25 HOW?? Low flow In Circuits 2 through 4 would result in Delta P in Circuit 1 being not much more than DP trasmitter setting of 10 psi 10 psi Cv= psi Cv= 22.2 Circuit 1 Cv = 15.9 OR SQRT(10/40) 30 psi Cv= psi Cv= 15.9

26 APPLICATION BASIC SYSTEM Circuit 4 10 psi DP Pressure Differential Sensor Δp=2 psi Coil # gpm Δp=4 psi 10 psi Cv= 31.3 Circuit 3 20 psi Δp=12 psi Coil #3 20 psi Cv= 22.2 Circuit 2 30 psi Δp=22 psi Coil #2 Circuit 1 40 psi VFD Pump Δp=32 psi Coil #1 Chiller 40 psi Cv= X 10 = 51 gpm

27 MANUAL BALANCE Circuit FLOW At DP Setting 10 PSID FLOW At Max DP Cv@10 PSID = 51 GPM 100% Cv@10 PSID = 58 GPM 100% Cv@10 PSID = 71 GPM 100% Cv@10 PSID = 100 GPM 100%

28 SO HOW MIGHT THIS PROBLEM BE FIXED

29 System Corrections INCREASE SENSOR SETTING

30 APPLICATION BASIC SYSTEM Circuit 4 10 psi 20 Pressure Differential PSI Sensor Δp=2 psi Coil # gpm Δp=4 psi 10 psi Cv= 31.3 Circuit 3 20 psi Δp=12 psi Coil #3 20 psi Cv= 22.2 Circuit 2 30 psi Δp=22 psi Coil #2 Circuit 1 40 psi VFD Pump Δp=32 psi Coil #1 Chiller 40 psi Cv= X 20 = 71 gpm

31 System Corrections MOVE SENSOR

33 System Corrections ADD MULTIPLE SENSORS

35 System Corrections REDUCE FRICTION LOSS IN DISTRIBUTION PIPING

37 Pipe sizing Criteria for sizing pipes are different from country to country but all have a common basis: Using small pipe sizes decreases the investment cost but increases the pumping costs, circulation noise and Dp variations with the load. Low circulation velocities cause deposit of heavier particles that can cause clogging by accumulation. Particle transport creates other problems for equipments where high circulation velocities are used. Consequently the generally used procedure consists in selecting the smallest pipe size that satisfies both a maximum linear pressure drop and a maximum velocity: Max linear pressure drop is around: Scandinavia Eastern Europe Western Europe China USA 100 Pa/m Pa/m Pa/m Pa/m 400 Pa/m Max velocities are usually between 1 and 2 m/s. For noise reasons, a maximum velocity of 0.5 m/s is considered inside residential buildings in some countries. In USA, the maximum velocity can be up to 3 m/s in large pipes. 37

38 WHAT WOULD BE THE WORST POSSIBLE SOLUTION TO THE PROBLEM

40 WHY NOT ELIMINATE THE BALANCE VALVE?

41 ASSUMPTION NO. 1 NO CHANGE TO THE CONTROL VALVES SAME Cv and DESIGN PRESSURE DROP

42 APPLICATION BASIC SYSTEM Circuit 4 8 psi DP Pressure Differential Sensor Δp=0 psi Coil # gpm Δp=4 psi 8psi Circuit 3 18 psi Δp=0psi Coil #3 18 psi Circuit 2 28 psi Δp=0 psi Coil #2 Circuit 1 38 psi VFD Pump Δp=0psi Coil #1 Δp= 19 psi Chiller Δp= 19 psi 38 psi 50 X 19 = 218 gpm

43 Assumption No 2 SIZE THE CONTROL VALVE TO COMPENSATE FOR THE REQIUIRED PRESSURE DROP OF BALANCE VALVE AT DESIGN FLOW

44 APPLICATION BASIC SYSTEM Circuit 4 8 psi DP Pressure Differential Sensor Δp=0 psi Coil # gpm Δp=4 psi Cv=50 8psi Circuit 3 18 psi Δp=0psi Coil #3 Δp= 14 psi Cv= psi Circuit 2 28 psi Δp=0 psi Coil #2 Δp= 24 psi Cv=20.44 Circuit 1 38 psi VFD Pump Δp=0psi Coil #1 Chiller Δp= 34 psi Cv= psi 16.2X 8 = 46 gpm

45 Why Not EliminateThe Balance Valve? Assumption No 1 No change in the design pressure drop of the control valve will result in immediate overflow at coils closest to the pump and potential underflow in coils further from the pump especially at start up. Other issues might include low delta T and noise due to excessive velocities. Assumption No 2 A change in design pressure drop of the control valve in theory could balance the system at design flow but we still experience issues with starving coils closest to pump under part load. Although in theory it is possible to balance by changing design pressure drop/ Cv of each control valve, the reality is that most manufactures do not offer multiple Cv s to match design flows. Every control valve would have a different Cv.

46 SO WHAT OTHER OPTIONS MIGHT WE HAVE?

47 AutoFlow AUTOFLOW

48 APPLICATION BASIC SYSTEM Circuit 4 10 psi DP Pressure Differential Sensor Δp=2 psi Coil # gpm Δp=4 psi 10 psi Cv= Variable Circuit 3 20 psi Δp=12 psi Coil #3 20 psi Cv= Variable Circuit 2 30 psi Δp=22 psi Coil #2 30 psi Cv= Variable Circuit 1 40 psi VFD Pump Δp=32 psi Coil #1 Chiller 40 psi Cv= Variable 31.6 x 10 = 100 gpm

49 Automatic Balancing

53 Commercial - small part but key effect Balancing valves cost less than 1% of project cost yet massively effect performance of all major equipment Chillers Cooling Towers Pumps Control System AHUs & FCUs Air Duct 2 way valves Pipe & General fittings Contracting Labour Consulting Balancing Valves

54 WHAT HAVE WE STILL OVERLOOKED?

55 VALVE AUTHORITY

56 WHAT IS VALVE AUTHORITY? WHAT HAPPENS IF WE HAVE POOR VALVE AUTHORITY?

57 Valve Authority - A A = the ratio of the ΔP across the control valve when it is wide open as a percentage of the ΔP across total branch or control valve closed The higher the authority the better - ASHRAE Systems Handbook and most control valve manuals recommend that it be 25 to 50% for HVAC applications

58 2-WAY CONTROL VALVE AUTHORITY Control valve authority should not be lower than 0.25 Control valves and controllability

59 STANDARD APPLICATION WITH 2-WAY VALVES AND MANUAL BALANCE VALVES Circuit 4 10 psi DP Pressure Differential Sensor Δp=2 psi Coil # gpm Δp=4 psi Authority A = 4/10 = 0.4 Circuit 3 20 psi Δp=12 psi Coil #3 A = 4/20 = 0.2 Circuit 2 30 psi Δp=22 psi Coil #2 A = 4/30 = 0.14 Circuit 1 40 psi Δp=32 psi Coil #1 A = 4/40 = 0.1 VFD Pump Chiller

60 Sizing with Authority Method Circuit 4 10 psi DP Pressure Differential Sensor Δp=2 psi Coil # gpm Δp=4 psi Authority A = 4/10 = 0.4 Circuit 3 20 psi Δp=6 psi Coil #3 Δp= 10 psi A = 10/20 = 0.5 Circuit 2 30 psi Δp=16 psi Coil #2 Δp= 10 psi A = 10/30 = 0.33 Circuit 1 40 psi Δp=26 psi Coil #1 Δp= 10 psi A = 10/40 = 0.25 VFD Pump Chiller

61 Sizing with Authority Method If we sized control valves for proper valve authority We may still not achieve design flow on coils closest to pump, even with AutoFlow

62 Sizing with Authority Method BUT We achieve approximately 90% heat transfer under worst conditions We maximize pump savings by turning down pump head under low flow conditions from 40 psi to 10 psi We actually improve valve authority in most circuits at part load I assumed some one would actually go through the process of calculating Cv s for each control valve

63 Alternative ALTERNATIVE PRESSURE INDEPENDENT CONTROL

64 Total Flow Control Match the flow to the load in every heat transfer device at all operating points, without excess flow Eliminate Low Delta T s Maximize Turndown on V/S Pumps Eliminate Flow Starvation at Coils Minimize Energy Usage

65 Pressure Independent Control Technically the best solution for controllability Regulator combined valves constitute an excellent alternative

66 PICV Overview H1=H2 H2 L2 L1 L3 H1 Ball Valve Design

67 50% Flow = 80% Heat Transfer

68 Dp stabilization on a control valve CONVERT ATC TO PRESSURE INDEPENDENT YC DA 516

69 Differential pressure controllers In order to preserve the authority of the control valves, limit noise close-off problems, a Dp controller is installed at the start of circuits.

70 70 Dp control at the inlet of buildings (sub-stations) YC DA 516 Sub-stations

71 Dp control per branch/floor 71 YC One Δp controller for each branch DA 5156 Excellent performance/cost ratio.

72 Forces in inline Dp controller Sm p1 p2 Fs p2 p3 p1 - pressure upstream the load p2 pressure downstream the load (taken inside the DP controller) p3 pressure downstream DP controller Fs spring force Sm diaphragm surface / area Equation: Δp = p1 p2 = Fs / Sm Only the seat moves, while plug is fixed, so p3 does not play the part in equation

73 Production and Distribution PRODUCTION AND DISTRIBUTION

74 Compatibility between flows false solutions "Pushing" the distribution pump as a reaction to complaints makes the problem worse It increases the flow incompatibility and therefore the mixing Supply water temperature increases further in cooling and decreases further in heating 150% 200% % 100% % A 150% 100% A 200% 50 Need of balancing in 74

75 Compatibility between flows false solutions Adding a production unit possibly solves compatibility issue but at the cost of an unnecessary production unit It is not a good solution because the problem is not a problem of lack of installed capacity, it is a problem of too high flow in the distribution 150% 150% 50% 0% 100% 150% 150% 150% Need of balancing in 75

76 Compatibility between flows false solutions Decreasing the set-point of the production unit can compensate for the incompatibility but at the cost of higher energy consumption 150% 150% % 50% C 100% A 150% 100% A 150% Need of balancing in 76

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