Low Performance of a plate heat exchanger

[Kdolph]

Hi everyone,

I have this doubt.

I have seen on the plant where I work these typical configurations of control on heat exchangers.

if STEAM is used to heat water or product, the control valve is placed in the steam inlet for all the heat exchangers I have seen (the majority are of plate and frame type, some shell & Tube).

if Cold Water or Chilled water is used to cold down a product, the control valve is placed downstream of the heat exchanger and not in the inlet of the cold water to the HE. My first question is why is this configuration preferred in this case, and not controlling the cold flow at the inlet??

My second question is related to that. It was recently installed a plate heat exchanger that has the function of cool down a product from 75°C to 70°C. The heat duty to do this is 30 kW, so it was installed a heat exchanger of a duty of 300 kW (not being used at the moment). It was used chilled water to cool down the product, at 2°C, and it was not possible to cool down the product more than 72°C. In this case
the control valve was place at the inlet of the Hx.
What could this happen? Will the situation improve if it is place the control valve downstream?

I really appreciate any help you can provide.

 

[Dougt115]

The basics without knowing more details.

Cold water valves downstream results in a more laminar flow in the heat exchanger than if the valve is upstream. Also restricting inlet water can result in air in the system as the system may drain faster than it fills.

Steam will condense as it cools so in this case you want the drain rate high in order to not have a build up of water. In steam heat exchangers you are also counting on a high flow rate and condensate is counter productive. Also the condensed water in turn acts like an insulation layer. Having the valve at the inlet lets the steam blow the exchanger clear.

Second part.
You have a flow rate issue or air in the system. You need to check that the system is not draining faster than it is filling. If this is the case then you have air in the system. Second make sure if it is a sealed system that the air has been bled out. Third if there is no air and it is a closed system then check the flow rate. If the flow rate is too slow then speed it up.