Fundamentals of Hydronic Design

Transcription

1 Radiant Based HVAC Systems

2 Dynamic Pressure

3 This educational material is copy written by Robert Bean, R.E.T., All Rights Reserved. If you wish to use this presentation for non commercial or for profit purposes, please contact for details and restrictions. Portions of this presentation are copy written by others including materials copy written 2005, by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This material may not be copied nor distributed in either paper or digital form without permission. Some slides contained animations in the original.ppt format which have been eliminated in the conversions to Adobe s.pdf format.

4 Dynamic Pressure Velocity Pressure Created with fluid circulation High to low pressure Differential Pressure Pressure measured across two points TDH Total Dynamic Head, measured across the circulator Head Pressure (Friction Head) Resistance to flow

5 Dynamic Pressure The Circulator Impeller Rotates System Fluid Not a Pump High Pressure to Low Pressure Water molecules rotated

6 Dynamic Pressure Pump Power Pump Efficiency Affinity Laws NPSH

7 Dynamic Pressure Pump Power The theoretical power to circulate water in a hydronic system is the water horsepower (whp) m, mass flow of fluid, lb/min at a specified temperature Δh = total head, ft of fluid 33,000 = units conversion, ft lb/min per hp m h 33,000 = whp

8 Dynamic Pressure Pump Efficiency Determined by comparing the output power to the input power: 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

9 Dynamic Pressure Pressure Drop Corrections Applied to Water for Glycol Solutions 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

10 Dynamic Pressure Pump Efficiency which is why for oversized circulators and improperly designed variable speed injection systems the warmer it gets the less efficient the system. 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

11 Dynamic Pressure Affinity Laws Flow (capacity) varies with rotating speed N (i.e., the peripheral velocity of the impeller). Head varies as the square of the rotating speed. Brake horsepower varies as the cube of the rotating speed. 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

12 Dynamic Pressure - Affinity Laws 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

13 Dynamic Pressure NPSH Net positive suction head (NPSH) (available) head at the pump inlet greater than the vapor pressure (expressed as head) at the operating temperature of the liquid being pumped.

14 Net Positive Suction Head h p + h z h vpa -h f = NPSHA t p h vpa h p h z h p = absolute pressure on surface of liquid that enters pump, ft of head h z = static elevation of liquid above center line of pump (h z is negative if liquid level is below pump center line), ft h vpa = absolute vapor pressure at pumping temperature, ft h f = friction and head losses in suction piping, ft

15 Dynamic Pressure Path of least resistance Closure increase differential across valve. Flow searches for a path of lesser resistance.

16 Dynamic Pressure Path of least resistance Closely spaced tee s generates little or no differential pressure. Flow bypasses fittings

17 Basic Fluid Fundamentals Dynamic Pressure Path of least resistance Reduction in diameter increases differential pressure across length of pipe Flow searches for path of lesser resistance.

18 Dynamic Pressure Thermo siphon Ghost flow, T air convection cools fluid in pipes t Convection Buoyancy Hot to cold q s + q p + even if p 0, q s will be +

19 Pressure Summary Static Weight of Fluid Expansion Temperature Dynamic Flow

20 Point of No Pressure Change Gil Carlson

21 Point of No Pressure Change Initial Cold Fill Static Pressure Radiator Non Compressible Compressible Circulator Air Charge Fill Pressure Boiler

22 Point of No Pressure Change The Parking Lot for Expanded Fluid If air charge exceeds static pressure fluid can t enter so relief valve will blow. If static pressure equals air charge and the tank is sized correctly there will be a place for the fluid to expand the parking lot. If static pressure exceeds air charge, tank will fill with fluid during system fill, there will be no room for expanded fluid, relief valve will blow

23 Possibilities Location, Location, Location S S S S Circulate away from relief valve and expansion tank Circulate away from relief valve but towards expansion tank Circulate towards relief valve and expansion tank Circulate towards relief valve but away from expansion tank

24 What would the gauges say with the circulator on/off????????????????? S S S S???? Circulate away from relief valve and expansion tank Circulate away from relief valve but towards expansion tank Circulate towards relief valve and expansion tank Circulate towards relief valve but away from expansion tank

25 Point of No Pressure Change Regardless of where the tank is located it will bear the weight of the water above it - measured in psig. The static pressure in either of these locations will equal. or Off

26 Point of No Pressure Change Regardless if it is hot or cold the weight of water is what it is. Hot or cold the static pressure is the weight of the water & since it can t* be compressed the operating pressure must change when the fluid is heated. Off

27 Point of No Pressure Change For fluid to move there has to be differential pressure. Without a change in pressure between the inlet and outlet there can be no flow in fluid. Off

28 Point of No Pressure Change For fluid to move there has to be differential pressure which occurs when the impeller is turned. Fluid will flow from a high pressure to a low pressure. On

29 Point of No Pressure Change Once again the weight of the water (psig) on the tank can t change but there has to be a pressure change to move the fluid Regardless of tank location (suction or discharge) the weight of the water would be equal. or Off

30 Point of No Pressure Change The static pressure can t be changed but there has to be a change in pressure to move the fluid Ask what will happen to the pressure if the tank is located on the suction or discharge side other circulator? or?? On

31 Point of No Pressure Change The static pressure can t be changed but there has to be a change in pressure to move the fluid Is it possible in a closed loop system to raise the pressure by pumping water into the tank? If so, ask where would the water come from & what replaces the water taken from the pipes?? On can you actually pump water into the tank?

32 Point of No Pressure Change The static pressure can t be changed but there has to be a change in pressure to move the fluid Is it possible in a closed loop system to lower the pressure by sucking water out of the tank? If so, ask where would the water go & what replaces the? water taken from the tank? On can you actually suck water out of the tank?

33 Point of No Pressure Change The static pressure can t be changed but there has to be a change in pressure to move the fluid Ask what happens if the tank is located on the suction or discharge knowing with pump off the gauges will be equal but with it on they will be different.?? Off

34 Point of No Pressure Change The static pressure can t be changed but there has to be a change in pressure to move the fluid With circulator off the gauges will be equal but with it on they will be different.?? Off On

35 The Relationship Between Static, Expansion and Dynamic Pressure

36 Why is this important? Point of No Pressure Change Since its not possible for fluid to be pumped in or out, it can enter the tank only through temperature and fill pressure. Off Off

37 Pumping Away With circulator off, the weight of the water (static pressure) will be equal on the pressure gauges Off

38 Pumping Away When the circulator turns on there will be a difference in pressure created and since the weight of the water can t be changed the change in pressure will show up on the discharge gauge. On

39 Pumping Away As soon the circulator is switched off the discharge gauge will once again measure the weight of the water (static pressure). Off

40 Pumping Towards With the circulator switched off the discharge gauge will again measure the weight of the water (static pressure).

41 Pumping Towards With the circulator switched on the pressure change can t change the weight of the water at the tank so it must show in the suction gauge resulting in a lower suction pressure. On

42 Pumping Towards With the circulator switched off both gauges will once again measure the weight of the water (static pressure). Off

43 Differential Pressure 25 psig Suction Side of Circulator Discharge Side of Circulator Differential Pressure P 15 psig Constant Static Pressure P

44 Differential Pressure 25 psig Suction Side of Circulator Differential Pressure Discharge Side of Circulator 20 psig P 15 psig Constant Static Pressure P 10 psig

45 Differential Pressure When there is insufficient static pressure & pumping towards the expansion tank! 20 psig Suction Side of Circulator Differential Pressure Discharge Side of Circulator 15 psig Correct Adjusted System Fill Pressure P 8 psig Incorrect Pressure P -2 psig

46 Cavitation occurs when P 1 Lowering the pressure P valve Lowers the boiling point Liquid flow to vapor to liquid P 3 P 2 P + min Implode (vs explodes) Creates a cavity cavitation P - cav Results: P 3 Oxygen induction Air locks P 2 Noise P 1 Damage ex. Cavitation Through a Control Valve

47 Cavitation There can be sufficient static pressure at the top of the building so long as there is no flow. When the circulator starts moving water across a valve or fitting with sufficient pressure differential pressure, it is possible to drop the pressure from positive to negative.

48 Possible Cavitation Solution Raise the static pressure from 15 psi to 25 psi but it requires larger tank With a 30 psi safety valve and a system static of 25 psig there is little expansion range so the parking lot size has to be increased ie: more room.! 30 psig 25 psig On

49 Possible Cavitation Solution Slow down the flow which decreases the differential pressure across the system but it reduces the average fluid temperature and possibly the heat output from the system. Lower flow velocity encourages air locks and noise. Separation of air occurs at the lowest pressure and highest temperature 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted with permission from ASHRAE Applications Handbook. This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission.

50 Possible Cavitation Solution The fluid temperature could be reduced but it the heat output may be influenced. May or may not be a problem.

51 Possible Cavitation Solution The tank and or circulator could (must be ) relocated which requires time, overhead and material cost. relocate

52 The best solution for dealing with cavitation is do it right - at the beginning calculate static pressure, pre charge tank, pump away.

53 Differential Pressure? Which choice will give you positive pressure at the highest point? Suction Side of Circulator Discharge Side of Circulator Differential Pressure Positive highest point 5 psi +/- Constant Static Pressure P (25 ft / 2.31 ft/psi) + 5 psi = 16 psig Suction Side of Circulator Differential Pressure Discharge Side of Circulator Calculate Static Pressure Pre charge P Tank? Constant Static Pressure Pump Away

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