Steam Blocked: What Happens to Temperature at 3.8 bar?

[nematnejad]

If i am supplying steam at a pressure of 3.8 bar (temperature at this pressure is 142 C) and the steam flow gets blocked. Does the temperature of the steam increase at the same pressure?

 

[russ_watters]

Welcome to PF!

No, I don't think so -- but tell me your thought process: Why do you think it might?

 

[nematnejad]

Thank you very much for your reply.

I am working with plastic foam processing. We use a machine called steam-chest molder. Very common products molded with this machine are the EPS products. Basically the machine uses steam to melt plastic particles and sinter them together to form three dimensional product.

Now as i am supplying steam the flow of steam gets blocked due to the resistance from these beads. So i see that at a pressure of 3.8 bar (supplied steam), the temperature at the steam entrance reaches around 170 C. As it passes through the mold cavity where it suffers pressure drop due to the resistance from the small plastic particles the temperature at the exit of the mold is 120 C (This is obvious due to the decrease in pressure).

However I am not sure why the temperature at the entrance is increasing as it damages the surface of the product due to excess temperature. I was thinking that the resistance causes increase in pressure at the entrance. However i still noticed the same pressure of 3.8 bar but the temperature was very high of 170 C.

I am not sure if there is some thermodynamic issue with enthalpy and latent heat.

 

[Q_Goest]

I could imagine a couple different reasons why this would happen.
1) How does the steam get heated? If heated by a constant heat flux device such as an electric heater, then a decrease in flow rate will increase the temperature coming out of your heater. Note that an electric heater will simply get hotter as you decrease flow rate, making your steam hotter.

2) Another possibility is there's thermal mass in your system between the steam source and plastic sintering processes that gets hotter over time, absorbing heat from the steam so that eventually the temperature comes to some thermal equilibrium but perhaps during this process you're referring to you don't quite get to thermal equilibrium and your steam gradually gets hotter.

Perhaps you should explain a bit more about your process, especially upstream of the plastic where you're creating and heating the steam. I don't see any way a decrease in flow rate should otherwise cause an increase in the inlet temperature if inlet pressure remains constant. That seems to indicate that you're boiling water at some steady pressure and superheating it afterwards.

 

[nematnejad]

The steam is supplied from a boiler room at a constant pressure of 5 bar via 2 inch pipes. It enter a mold cavity via 2 inch insulated piping. The mold cavity where the plastic material is molded is 15 cm x 6cm x 5cm. The steam enters this cavity through small ports on the surface of the mold.

However the steam from the 2 inch insulated pipe enters a 50 cm x 50 cm x 50 cm empty cavity prior to flowing into the small mold cavity where the plastic material will be molded. So there might be sudden increase in volume from 2 inch pipe to the large empty cavity.

 

[Q_Goest]

What's the temperature of the steam at the source?

 

[nematnejad]

approximately 145 C- 151 C

 

[russ_watters]

That's odd. 151C corresponds to the saturated steam temperature at 5 bar, so it doesn't make sense to see higher than that.

 

[Q_Goest]

Yes, at 5 bar absolute the saturation temperature is 151 C.
At 5 bar gauge the saturation temperature is 159 C.

You should specify gauge or absolute pressure.

Regardless, there's nothing that can cause the temperature to increase above that temperature if there's no energy being added to the steam. It doesn't matter if the volume changes. If the pressure drops, the temperature will only go down.