HYDRONICS
Accurate Flow Control with Enhanced System Performance
BY PETER BIONDO 50
selection of high efficiency equipment but also in the managed control of flow rates through each terminal unit. Terminal units include fan coils, air handlers, chilled beams, radiant heat emitters and convectors. Balancing valves and control valves typically manage flow through the terminal unit. Without good hydronic balancing, flow will vary and is difficult to control as valves open and close throughout the building. Flow control across a terminal unit
E
is a problem with some HVAC systems. Overflow will raise the average temperature of the terminal unit and resulting thermal output. Underflow lowers the average temperature, and the terminal unit may not meet load demand. Hydronic systems are subject to dynamic pressure changes when valves open and close. Because of these pressure changes, flows and temperatures in the building are often uneven. The situation worsens at low and medium loads and can cause unwanted system cycling. Boilers and chillers end up running more often. “Out of balance” means
“out of pocket” for building owners. The standard installation will
experience differential pressure changes across the control valve, independent of the desired response.
nergy efficiency improvements within an HVAC system can be achieved not only by the
A controller signal may have to “hunt” for the flow, as differential pressures change and effectively delay the response time of the terminal unit. Operating efficiency is tightly linked to stable flow rates and a correlating response to a control signal. The ideal hydronic control would be represented by a balancing device and a control valve that achieve the desired flow regardless of any pressure fluctuations. The pressure independent control valve (PICV) combines the features of a differential pressure regulator, a control valve and a balancing valve. PICVs have solved the vexing
The pressure independent control valve (PICV) combines the features of a differential pressure regulator, a control valve and a balancing valve.
Because of this control, valve authority in the PICV is 100%. Three design parameters to
issue of regulating accurate flow over a wide range of pressure variations. The PICV is a two-way valve that combines control and balance in one valve. Actuators are available for common control signals, including on/off and proportional. Some models employ a dial for selecting the flow rate in the field. The big advantage that the PICV has over other balancing devices is the operation of the differential pressure regulator. All pressure changes across the PICV are absorbed by a differential pressure regulator, which maintains a constant differential pressure across the control valve.
consider when selecting the appropriate PICV are maximum design flow rate, available minimum differential pressure and possible maximum differential pressure. To regulate the flow correctly, the valve needs to operate within a range of differential pressure, bound on the bottom by the minimum differential pressure available and on the top by the maximum differential pressure possible. When designing, be sure that the pump keeps the valve within these parameters. To keep costs in line, choose the smallest possible valve that achieves the maximum design flow rate. These design parameters will help you select the best PICV for your application.
e Continued on p 52
phc october 2011
www.phcnews.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88