Variable flow primary pumping systems utilize a variable capacity cooling system and a variable flow water pumping system to modulate cooling and water flow to meet chilled water needs and save operating energy costs.
- Cooling capacity is modulated with variable capacity compressors based on the desired leaving water temperature.
- Water flow is modulated with VFD controlled variable flow primary pumps based on the differential pressure
across the water system.
The variable flow primary pumping system is the most economical and energy efficient chilled water system available.
To understand the benefits of VFP system, first we will see the concept of constant primary flow system.
Constant Primary- Variable secondary Flow (Decoupled system)
- Pumps pushing water to the chiller is known as primary flow circuit, whereas the pumps supplying water to the cooling coils in a building space are called as secondary flow circuit. Accordingly pumps are designated as primary pumps (located upstream of chiller) & secondary pumps (located downstream of chiller).
- The above schematic shows the arrangement of equipment’s for so called “decoupled system”
- The decoupled system uses constant water flow through each chiller evaporator & variable water flow through each cooling coil to satisfy space loads.
The system consists of
- A constant speed & constant volume primary pumps : this pumps keep supplying constant flow to the chillers.
- Two way control Valve: to regulate the amount of chilled water flow through the cooling coils.
- A variable flow distribution pump to serve the coils.
- A bypass to hydraulically decoupled the primary & secondary system.
- As each two way valve adjust the flow of chilled water through the coil to satisfy existing load, the variable flow distribution pump responds by regulating the amount of chilled water delivered.
- Water flows through bypass in either direction as needed to balance the system.
Primary flow must always be equal to or greater than secondary flow
Variable Primary Flow System
In VPF System, the water flow varies thought the entire system, that is through the evaporator of chiller as well as through the cooling coil .
The system consists of
- A variable flow primary pumps: Water flow is modulated with VFD controlled variable flow primary pumps based on the differential pressure across the water system.
- Two way control Valve: to regulate the amount of chilled water flow through the cooling coils.
- A bypass line equipped with control valve & flow meter to ensure that amount of floe that returns to the chiller never falls below the minimum limit.
Benefits of VPF System compared to constant Primary Flow system
- Little Low installation Cost: Both the system have almost similar arrangements except for secondary pumps are eliminated in VPF system instead a higher capacity VFD pumps are used only in primary circuit which regulates the flow in entire system. Other accessories like flow meter , bypass control valve, DP sensor, chiller isolation valve are some additional cost in VPF but still roughly overall 5% cost can be saved in initial investment.
- High energy saving: Variable flow primary pumping systems save energy compared with constant flow pumping systems during part load operation because water flow is reduced and less pump energy is required. Also lesser pumping power required as overall system head loss reduced. Around 15% net saving in pumping power is possible with VPF. In addition, with reduced water flow during part load operation, less compressor energy is required. So Overall 20-30% operating cost saving is possible.
- Less plant room Footprints: Since the secondary pumping system is eliminated, their installation space are saved & can be utilized for some other purpose.
- Better response to Low Delta T syndrome: Low delta T is the situation when because of several potential chilled water or air side issue, system ends up pumping more water than actually required for the building load, thus wasting energy. To compensate this, additional pump has to be turned on in constant primary flow system while the same can be achieved by just speeding up the already running pump in case of VPF.
Challenges
- Avoid using VPF where system chilled water temperature is critical such as Clean room or process application
- In the systems where only three way valves are used meaning that the system flow rate does not vary a VPF system can not be used.
- Do not use VPF if it is unlikely that the operator will run the plant as designed.
- System must be able maintain a minimum flow rate through the chiller, have quick unloading cooling capacity and have condenser head pressure control.
- Require more robust (complex & calibrated) control system.
- Require coordinated controls of chillers, isolation valves & pumps in sequencing.