Interactivity-Compatibility. TA Hydronic College Training

Transcription

1 Interactivity-Compatibility TA Hydronic College Training

2 Hydraulic interactivity B Both valves are open. What becomes the flow if one of these valves is closed? p AB A H Head [ft] p AB = Pump head H 1 1//2 The flow in each unit only depends on p AB (= the pump head) As the pump head does not vary so much with the total flow, the flow in one unit is not much affected by the opening/closing of the other unit Flow [%] 2

3 Hydraulic interactivity B Both valves are open. The pressure drop in the load is high compared to pressure drop in units 1 and 2. What becomes the flow if one unit is closed? p AB Load p load A H Head [ft] Pump head H With most of the pressure drop in the load, the total flow does not change much when closing unit 2. So that the flow in unit 1 is highly affected when unit 2 is closed (almost doubled) p AB = H p load Flow [%] 3 1 1//2

4 Hydraulic interactivity Hydraulic interactivity does happen even if each unit has its own pump. The degree of interactivity is however smaller in this case. Load 1 2 Load 4

5 Hydraulic interactivity H 1 H 2 B Both valves are open. What becomes the flow if one of these valves is closed? 1 2 p AB 140,00 p unit1 (= p unit2 ) A Head [ft] 120,00 100,00 Pump head H 1 (= H 2 ) The flow in each unit only depends on p AB 80,00 60,00 Circuit 1 // Circuit 2 p AB is constant (equal to 0). the flow in one unit is not at all affected by the opening/closing of the other unit. 40,00 20,00 0,00 Circuit 1 = H 1 p unit Flow [%] 5

6 Hydraulic interactivity H 1 H B A p AB Load With most of the pressure drop in the load, p AB varies steeply (compared to the p of circuit 1 and 2) and the total flow does not change much when closing unit 2. Both valves are open. The pressure drop in the load is high compared to pressure drop in units 1 and 2. What becomes the flow if one of them is closed? Head [ft] 140,00 120,00 100,00 80,00 60,00 40,00 p AB = p load Circuit 1 = H 1 p unit1 Circuit 1 // Circuit 2 Pump head H 1 (= H 2 ) So that the flow in unit 1 is highly affected when unit 2 is closed (almost doubled). 20,00 0,00 p unit1 (= p unit2 ) Flow [%] 6

7 Hydraulic interactivity - definition Hydraulic interactivity between different units happens when several units in parallel share a common hydronic resistance Common resistance Symptoms: Any variation of flow through one circuit affects the flow in the other circuits 7

8 Hydraulic interactivity driving parameters B The larger the hydronic resistance of the common load, the larger is the interactivity between circuits Common resistance p AB The larger the p of the common load (w.r.t. the p of the circuits), the larger is the interactivity between circuits A The smallest the design flow of one circuit, the more it is interactively affected by the others 8

9 Hydraulic interactivity - criterion B 100 q i max [%] Common resistance q a 4b q2 q3 q4 p AB Φ=0.9 Φ=0.7 q T A Let Φ = p AB max / pump head of circuit n and λ = q n d /q T max λ = q i d /q T max Φ=0.5 Φ=0.3 Φ=0.1 If we accept a maximum interactive flow variation of 20%, p common < 30% of the lowest pump head in the concerned circuits 9

10 Hydraulic interactivity easy to remember criterion 1 2 common load p common < 0.3 H smallest 10

11 Hydraulic interactivity The boilers 1 and 2 are a common resistance in series for distribution circuits and. ts A qs qs Circuits affects the flow in circuit and reciprocally. tr tr Boilers are supplied at variable flow which is not acceptable for standard boilers. 1 2 q p B 11

12 A bypass solves interactivity problems qs qs The bypass between A and B keeps the differential pressure between A and B close to zero ts A No interactivity between circuits and No Interactivity between boilers 1 and tr tr Constant flow in each boiler qp B 12

13 A bypass solves interactivity problems qs qs The bypass between D and E keeps the differential pressure between D and E close to zero No interactivity between circuits D No Interactivity between boilers Constant flow in each boiler qp E 13

14 Hydraulic interactivity 54 F 43 F? This design is not recommended. Chillers are interactive. When a second chiller is switched on, the total flow does not change so much because most of the pressure drop is in the distribution. Then suddenly, the flow in the first chiller drops. As the chiller power does not drop instantly, the temperature in the evaporator can reach the freezing point. 14

15 A bypass solves interactivity problems 43 F A bypass avoids interactivity between chillers A second pump, sized for the total flow is required. As the pump cost depends essentially on its nominal flow, designers sometimes prefer an alternative design. 15

16 Differential pressure relief valve A differential pressure relief valve in the bypass solves interactivity as the pressure drop between A and B does not depend on the flow. The supply temperature in the distribution is constant. 43 F 43 F 16

17 Sizing of differential pressure relief valve Sizing of the differential pressure relief valve must be done for the max. flow that can go through the bypass 43 F 43 F The max. flow going through the bypass is determined by finding the max. difference between the production and distribution flows Cooling output - = Chiller flow Distribution flow (for prop. control) Bypass flow 17

18 Hydraulic interactivity different cases Interactivity between/with production units t s A qs tr qs tr Problem: Flow variations in production units that do not allow it Too fast flow variations in production units that are controlled in cascade qp B Solutions: Decoupling bypass Differential pressure relief valve in bypass 18

19 Hydraulic interactivity different cases Interactivity between circuits when balancing Problem: Flow adjustment in one circuit changes the flows in all other circuits Solutions: Decomposition in hydronic modules Use of a systematic balancing methods based on the proportion rule (Compensated method, TA-Balance) Balancing facilitated by differential pressure controllers on branches 19

20 Hydraulic interactivity different cases Interactivity between circuits in variable flow systems by action of control valves Problem: Unstable control due to differential pressure variations induced by opening and closing of control valves Solutions: Size control valves to get a good authority (= make control valve Dp large compared to common Dp) Use differential pressure control to protect control valves from differential pressure variations 20

21 Interactivity Is there any risk of interactivity? Describe what could happen. Pump head is 15 psi / 35 ft and distribution circuit pressure drop is 15 ft. Is interactivity acceptable? Which solution could you advise to avoid interactivity? 21

22 Interactivity B For which circuit(s) is interactivity acceptable? If not acceptable, what is the maximum flow deviation? Common resistance H=20 ft H=10 ft H=13 ft H=16 ft H=50 kpa q a 4b q 2 q3 q4 5 ft q T A Design flows are identical in each circuit. 22

23 Interactivity in real life Why is there a risk of interactivity? What should be the condition to make interactivity negligible? 23

24 Compatibility between flows A bypass between the production side and the distribution side avoids interactivity BUT Compatibility between flows has to be assured 24

25 Production and distribution flows Flow in production units are usually relatively well set, because of: published manufacturers' limits warranty Flow on distribution side is generally well above design value, because of: safety factors "what can do the most can do the least" approach at all stages 100% flow 150% flow In 90% of installations, flow in distribution is over 150% of the design value. Source: Investigation by Costic (French Research and Training Centre in HVAC), published in CFP Journal April-May

26 Compatibility between flows Boilers 100% 61 C 80 C A 180% 80% 72 C 176 F 161 F Distribution 61 C 142 F 180% 142 F Because the distribution pump is oversized, the distribution takes more flow than the production can provide. There is a mixing point in A between return water and supply water. The supply water temperature is lower than expected per design. 26

27 Compatibility between flows Chillers 11.8 C 53 F 6 C 100% A 150% 50% 150% 11.8 C 53 F Distribution 43 F 46 F 7.9 C Because the distribution pump is oversized, the distribution takes more flow than the production can provide. There is a mixing point in A between return water and supply water. The supply water temperature is higher than expected per design. 27

28 Supply water temperature drift versus overflow For a 43 F 54 F 75 F temperature regime Emission 100% 120% Emission 90% 80% 100% 70% 60% 80% 50% 60% 40% 30% 40% 20% 10% 0% Supply water temperature [ F] 20% 0% 0% 50% 100% 150% 200% Flow Decreased supply t effect - 18% Overflow effect + 10% 28

29 Compatibility between flows qs qs The problem of flow compatibility does not appear under all conditions: t s A At lower load everything seems to work fine. tr tr At high load (when peak power is needed) incompatible flows limit the power which is transmitted from the production to the distribution. When q p < Σq s, the flow reverses in the bypass and return water is mixed with supply water. qp B 29

30 Compatibility between flows qs qs t s A tr tr When the production side is designed as a loop: q p B Flow incompatibility starts by affecting the last circuits. The problem of flow compatibility appears only at high load. 60% 100 % 40 % 60% qp > qs 20% q p 60% 60% 100 % 40 % 20 % 30

31 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 (decreases further in heating) 150% 200% 11.8 C 53 F 11.8 C 53 F 11.8 C 53 F 11.8 C 53 F 50% 100% 43 F 46 F 6 C 100% 7.9 C 6 C A 150% 100% 43 F 48 F A 200% 8.9 C 31

32 Compatibility between flows false solutions Decreasing (raising in heating) the set-point of the production unit can compensate for the incompatibility but at the cost of higher energy consumption Chiller manufacturers technical literature indicates extra energy use of approximately 2.2% per F that the chilled water supply temperature is lowered. 150% 150% 11.8 C 53 F 11.8 C 53 F 12 C 54 F 12 C 54 F 50% 50% 43 F 46 F 6 C 100% 7.9 C 3 C A 150% 100% 37 F 43 F A 150% 6 C 32

33 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% 33

34 Compatibility between flows the solution 100% Provide the design flow to all units. At the right supply water temperature. Do not take more flow than produced. A ~ 0% 100% 100% Need to balance: At each system interface, guarantee the flow compatibility thanks to balancing on the production side and on the distribution side. 34

35 Nothing in a straight bypass!!! qs qs qs qs qp qp NO valve! NO check valve! 35

36 Compatibility with constant flow distribution B A constant flow distribution combined with a production side controlled in cascade is an incompatible system by nature ts ts F 10 Balanced Unbalanced A 9 8 When the system is not balanced, it is not possible to deliver the installed capacity in design conditions. 43 F Load 36

37 Compatibility between flows (a) (b) 1. What are the compatibility conditions for designs (a) and (b)? 2. From the operation point of view, what is the difference between (a) and (b)? 3. When is design (b) recommended? 4. Is it possible to control the boilers in sequence so that the supply temperature (t s ) is 194 F and constant (Think about what could happen when 1, 2 or 3 boilers are working)? 37

38 Compatibility between flows (a) (b) 1. What are the compatibility conditions for (a) and (b)? 2. Is it possible in both cases to keep 43 F supply water temperature in the distribution circuit? 38

39 Compatibility in practice Which condition should be fulfilled to guarantee compatibility between flows? 39

40 Exercise solutions 40

41 Interactivity Is there any risk of interactivity? Describe what could happen. Yes, there is. When control valve V2 closes, the flow through the boilers decreases reducing their Dp. This gives a higher available differential pressure increasing the flow through valve V1. Pump head is 15 psi / 35 ft and distribution circuit pressure drop is 15 ft. Is interactivity acceptable? Interactivity is not acceptable: 15/35 = 0.43 > 0.3 Which solution could you advise to avoid interactivity? Add a bypass between production and distribution (and a second set of pumps) 41

42 Interactivity Design flows are identical in each circuit. B For which circuit(s) is interactivity acceptable? If not acceptable, what is the maximum flow deviation? Common resistance Circuit 1: 5/20 = 0.25 < 0.3 Interactivity is acceptable Circuit 2: 5/10 = 0.5 > 0.3 Interactivity is not acceptable a 4b Max flow deviation will be 39% Circuit 3: 5/13 = 0.38 > 0.3 Interactivity is not acceptable q 2 q3 q4 Max flow deviation will be 25% Circuit 4a and 4b: 5/16 = 0.31 q T A Interactivity is just at the limit of the criterion. It will lead to 20% flow variations in circuits 4a and 4b. It must be checked with the design engineer if this is OK for the equipment and circuit function. H=20 ft H=10 ft H=13 ft H=16 ft H=50 kpa 5 ft 42

43 Interactivity in real life Why is there a risk of interactivity? The boiler is a common resistance for the 3 circuits connected in parallel. What should be the condition to make interactivity negligible? H 1 H 2 H 3 p boiler /H 1 < 0.3 p boiler /H 2 < 0.3 p boiler /H 3 < 0.3 p boiler 43

44 Compatibility between flows (a) (b) 1. What are the compatibility conditions for designs (a) and (b)? (a) Design flows: q g q p q p 2 q s (b) Design flows: q g 2 q s q s1 q c1 q s2 q c2 44

45 Compatibility between flows (a) (b) 2. From the operation point of view, what is the difference between (a) and (b)? There is no difference w.r.t. breaking the interactivity. In both cases, circuits get disconnected in pressure from the distribution. Schemes (a) and (b) behave differently in case of incompatibility: with scheme (a), the last circuit get first affected; with scheme (b), compatibility must be ensured for each circuit independently of the others. 3. When is design (b) recommended? When the distribution length is large in design (a), the Dp of the pipe acts a common resistance for the 2 circuits. Circuit (b) with "distributed" bypass is then recommended. 45

46 Compatibility between flows (a) (b) 4. Is it possible to control the boilers in sequence so that the supply temperature (t s ) is 194 F and constant primary flow (Think about what could happen when 1, 2 or 3 boilers are working)? No. When only 1 or 2 boiler(s) work(s), there is a mixing in A as the production flow is reduced to 1/3 (2/3) of the total production flow while the distribution flow is constant. 46

47 Compatibility between flows (a) (b) 1. What are the compatibility conditions for (a) and (b)? (a) No compatibility condition, but chillers are interactive! (b) Design flows: q g q s 2. Is it possible in both cases to keep 43 F supply water temperature in the distribution circuit? Not in case (a), if chillers are not isolated when switched off. Not in case (b), when some chillers are not in operation. In this case, there is a mixing in A as the production flow is reduced to a fraction of the total production flow while the distribution flow is constant. 47

48 Compatibility in practice Which condition should be fulfilled to guarantee compatibility between flows? Design flows q g Σ q si q g q s1 q s2 q s3 q s4 q s5 q s6 q s7 48