In an energy efficiency initiative, independent control valves become an interesting alternative. Learn about some features of this type of system.
by Peter Biondo*
Energy efficiency improvements within the HVAC system can be achieved not only with the selection of high-efficiency equipment, but also in the control management of flow rates through each terminal. The terminals are fan coils, air controller, cooling beams, radiant heat emitters and convectors. Balance valves and control valves usually manage flow through a terminal. Without a good hydraulic balancer, the flow is variable and difficult to control, such as opening and closing valves throughout the building.
Terminal flow control is a problem with some HVAC systems. The overload will raise the average temperature of the terminal resulting in a thermal output.
The overload lowers the average temperature, and the terminal cannot meet the load demand. Hydronic systems are subject to dynamic pressure changes when valves open and close. Due to these pressure changes, flows and temperatures in the building are often irregular.
The situation is aggravated by low and medium loads and can cause an unwanted system cycle. Boilers and coolers end up running more often. "Off balance" means "out of budget" for homeowners. The standard installation will experience differential pressure changes through the control valve, regardless of the desired response.
A control signal will have to find the flow, as different pressure changes will effectively delay the terminal's response time. The efficiency of the operation is closely linked to the stable flow ranges and a related response to the control signal.
The ideal hydraulic control would be represented by a compensation device and a control valve that allows the desired flow to be reached, regardless of any pressure fluctuations. The independent control valve (PICV) combines the characteristics of a differential pressure regulator, a control valve and a balancing valve.
The standard installation will experience differential pressure changes by means of the control valve, regardless of the desired response.
PICVs have solved the annoying use of flow regulation through 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 button to select the flow rate in the field.
The great advantage that PICVs have over other balancing devices is the use of the differential pressure regulator. All pressure changes through the PICVs are absorbed by a differential pressure regulator that keeps it constant using the control valve. Because of this, the control of picv is 100%.
The three parameters designated to select the appropriate PICV are maximum flow rate, availability of minimum differential pressure and the possibility of maximum differential pressure. To regulate the correct flow, the valve needs to operate within a differential pressure range, linked at the bottom by the minimum available differential pressure and at the top by the maximum available differential pressure. During design, make sure the pump keeps the valve within those parameters. To maintain costs, choose the smallest valve possible that achieves the maximum flow range. These designated parameters will help you select the best PICV for your application.
Characteristics of PICVs
Features vary depending on the models manufactured. One benefit of all models is that they are compact; a compact valve takes the place of two. Something in common with all PICVs is that they have a pressure regulator consisting of a spring and diaphragm cartridge assembled. The differences are in the flow control mechanism. Some models have a control disc to adjust the flow; others use a ball-style valve.
Multiple types of flow can be found in one valve. Some of those models incorporate an adjustable field line to mark the amount of flow from the top and end. Those brands are closed. Obtaining them may require removing the actuator or removing the side gate; in others, the marks on the valve can be rotated and are legible in any position.
Some brands are in percentage values, while others are registered in gpm. Pressure point tests are available on most models. Additionally, to make them easy to design, PICVs offer a simplified balancing procedure. Some valves are not adjustable and are assembled in the factory; others are armed in the field. Both types only require that the differential pressure be checked through the valve to ensure proper flow.
PICVs are designated to operate within the specifications that most HVAC systems will require. Flow control starts with an o.5 gpm 1/2-inch valve and covers a flow range of up to 700 gpm with a 6-inch valve. PICVs operate over a wide range of differential pressure. The minimum differential pressure can be at least from 3 to 5 psi. The maximum differential pressure is up to 50 or 60 psi. Some models can operate with a pressure as high as 90 psi. Pressure flow varies per model from +/3% to +/-10%.
Depending on the model, working temperatures can range from 0 F to 250 F. The maximum working pressure for a PICV can be as high as 300 psi. Please refer to the models manufactured by the product specification.
PICVs with variable flow system
There is no doubt about the reasons why PICVs have gained great acceptance in the energy efficient market today. Using a variable frequency unit (VFD) for a control pump allows the pump to change speed based on differential pressure control at a reference point in the system. A variable system pumping speed will provide you with the right amount of flow to meet the change requirements. PICVs allow by themselves the energy savings that the VSD provides allowing the desired flow in each terminal.
The exact flow capitalizes on the benefits of the variable volume of pumping systems. Overflow is eliminated, increasing the available capacity of the plant and can minimize capital expenditures for additional capacity. The ability of PICVs to maintain a stable flow is useful for load changes from one site to another. The use of PICVs ensures that only the necessary amount of hot or cold water is sent to cooling and/or heating load all the time.
This article describes how the use of PICVs gives better results in efficiency and control. As I wrote this I realized how important it is to have a flow control design of a coil fan, air controller, cooling beams, radiant heat apparatus or convector. Ideally, every designer should make plans for energy efficiency with precise flow control. Installing PICV terminals to correct flow rate is a simple solution.
*Peter Biondo is the technical sales coordinator for Oventrop.
