Steam & Condensate Introduction
Continuous Process Application: Shell & Tube Heat Exchanger
Same as inlet pressure to steam trap
Steam pressure to Heat Exchanger
Let’s take a detailed look at a continuous process application using a control valve on a Heat Exchanger to heat a variable flow rate of water to a constant temperature. Cold water enters the Heat exchanger and hot water is discharged at an elevated temperature. The condensate load and pressure drop (differential pressure) across the steam trap are not constant. Therefore, it is important to select a steam trap that can handle high condensate loads at very low pressure drops, without significantly oversizing the steam trap during normal operation. A temperature control valve will modulate between an open and closed position to deliver the proper amount of steam to a heat exchanger to maintain the outlet water at a desired temperature. During this process, the steam pressure in the heat exchanger will vary depending on the flow rate of heated water produced. The higher the flow rate of water – the higher the steam pressure in the heat exchanger will be. Conversely, when water flow is reduced, steam pressure is reduced.
Inlet pressure to Control Valve
Steam Control Valve
Temperature Controller
Air Vent
Vacuum Breaker
Steam
Temp Sensor
Heat Exchanger
hot water outlet
F&T TRAP
cold water inlet
DIAGRAM 1 : Start-Up (Air Vents Open)
DIAGRAM 2 : Steam Enters (Trap Fully Open; Air Vents Closed) Since the water temperature is cold, the control valve is fully open to allow as much steam as possible to fill the heat exchanger. The steam trap must adjust to the high condensate load as the steam is entering and building pressure. This steam pressure in the shell of the heat exchanger pushes the condensate through the steam trap and into the return line.
On start-up, heat exchanger is filled with air which must first be discharged by the Air Vents to allow steam to enter for heating. Float & Thermostatic steam traps contain a separate thermostatic vent, and can discharge large volumes of air present during system startup.Additional air vents may be installed on the heat exchanger. The faster air is expelled, the faster steam can enter and heating can begin.
Air Vent CLOSED Steam Temperature Closes Air Vent
High Pressure
Same as Inlet pressure to Steam Trap Low Pressure
Inlet pressure to Control Valve
Air Vent OPEN Releases air during start-up
Steam pressure to Heat Exchanger
Air Vent Closed
Air Vent Open
Air
STEAM
STEAM
Trap CLOSED Air Vent OPEN Discharging Air on start-up
AIR
Air Vent CLOSED when steam is present
STEAM
AIR
condensate
condensate
Trap Fully OPEN Discharging Condensate
Air
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