As name suggest stall is the condition of heat exchanger where it ceases to perform work of heat transfer due to no flow or uneven flow of steam in the heat exchanger. lets discuss, What is Stall and how to prevent it in Heat exchangers?
In steam network, high pressure steam is supplied to the heat exchanger and condensate is continuously removed to perform heat exchange. For condensate removal steam traps are used. The pressure difference between inlet steam and condensate keeps the condensate evacuate from the heat exchanger to perform effective heat transfer. In case low temperature heating typically below 100 deg C, Differential pressure of steam and condensate will reduce and sometime it becomes negative. When there is no difference between steam inlet and condensate outlet pressure stalling will occur. below table shows condition of stall
|Stalling Condition||Condensate Outlet pressure|
|NO||Lower than Steam inlet pressure|
|Yes||Equal to Steam inlet pressure|
|Yes||Higher than Steam inlet pressure|
Effect of stall in Heat exchanger ?
1 No Heat transfer and loss of productivity : If stalling occurs in heat exchanger heat transfer will ceases to continue. This impact the purpose of installation of heat exchanger and productivity of the heat exchanger is compromised. Normally people bypass steam trap at this point and most of the time they forget to tack steam trap back into line. Bypassing of condensate not only increases steam consumption but also increases steam consumption.
2 Uneven Heat transfer and quality loss : With stalling heat transfer may become uneven especially where steam and condensate pressure keeps on varying from positive to negative. During uneven heat transfer may product gets quality issue and operator is confused for taking steam trap in line for condensate and steam consumption optimization.
3. Non utilization of Heat transfer area : Stalling will block effective heat transfer area of heat exchanger. Heat exchanger is unutilized and some time it has been observed that people add area or additional heat exchanger to over come this problem.
4. Water Hammer : As condensate will accumulate inside heat exchanger during stalling condition, it will not allow steam to flow properly causing water hammer inside heat exchanger. Water hammer may create abnormal knocking sound and leakages in the heater exchanger tubes.
5. Heat exchanger leakage : Due to water hammer and water corrosion, there is chances of heat exchanger leakage and failure.
How to Prevent Stall in Heat exchanger ?
There can be various method to eliminate stall in heat exchanger, Lets discuss
1 Keep Steam pressure above atmospheric pressure and use gravity for condensate withdrawal :
This is the most economical and suggested method of stall prevention, in this method we need to keep steam pressure above atmospheric pressure, add vacuum breaker and create static leg above trap usually above 0.5 Mtrs to keep condensate flowing. Refer below diagram fig 1 for better understanding, investment requirement in this arrangement is very less and only vacuum breaker and trap head to be adjusted. This type of arrangement can also be used along with standard setup of steam trap in case where there is stalling only during startup of the process, Like heating from atmospheric to higher temperature and then kept it operating at higher temperature, in this case during startup static head trap is used to evacuate condensate from the system, latter on after achieving temperature standard trap can be used. We should collect condensate from the steam trap and utilize it as boiler feed water
2 Use of condensate pump with balancing line
Use of condensate pump with balancing line is very common method of stall removal in large heat exchanger, fig 2 Shows the schematic diagram of condensate pump with balancing line, This system looks like PPPPU condensate system with a difference that exhaust is not vented to atmosphere and pressure is balanced with heat exchanger pressure. As the pressure is balanced condensate flows to the pumping trap and from where it is pumped to condensate line with the use of motive pressure. NRV is used for directional control of flow. Water is collected in reservoir and then fed to pump trap, In pump trap ball float is used and when it is filled with condensate motive steam is fed to the vessel. Due to steam pressure condensate will come out from outlet NRV or check valve. As steam pressurizes the vessel incoming NRV restrict the flow and flow is directed to condensate header. The condensate flows and then steam pressure inside vessel is released to reservoir. As steam pressure is released to reservoir pressure get balanced between reservoir, Vessel and heat exchanger and condensate will start flowing to vessel for next operation.
3 Use of Automatic Pumping Trap
Automatic pumping trap is popular for small capacity requirement, It is very compact version of condensate pump with balancing line. Refer fig 3 for schematic, The functioning of system shall be similar to condensate pump and reservoir system with differ as reservoir is replaced by line length between condensate and heat exchange. Balancing and transfer is made compact with combined in very small space. Automatic pumping traps required very less floor area and also it looks very complicated for installation and maintenance.