DIFFERENCE BETWEEN OPEN FEEDWATER HEATER AND CLOSED FEEDWATER HEATER

We were discussing “The basic concept of Rankine cycle in a steam power unit” in our recent post, where we have discussed the various components of steam power cycle and their basic operation too.

We have also seen the concept of regenerative Rankine cycle with feedwater heater in our previous post. We will discuss here the types of feedwater heaters with the help of this post.

Objective of using feedwater heaters is to improve the thermal efficiency of the steam power cycle i.e. Rankine cycle. Feedwater heaters are basically one type of heat exchangers and these feedwater heaters are used in steam power plant for preheating the feedwater of boiler by securing the heat energy from the extracted steam or bled steam from the turbine. are classified in two types i.e. open feedwater heaters and closed feed water heaters.

Let us first see here the open feedwater heaters

Open feedwater heaters are also termed as contact type feedwater heaters. In case of open feedwater heaters, extracted steam or bled steam coming from turbine will be mixed with feedwater in heater or heat exchanger. Mixture of bled steam and feedewater will leave the feedwater heater or heat exchange at a common temperature. 

Process will be controlled in such ways that discharge of feedwater heater or heat exchanger will be in the state of saturated liquid state.

Following figure displayed here indicates the regenerative cycle block diagram with two open feedwater heaters.

 

Let us see here the basic concept of open feedwater heater with the help of following image

Open feedwater heaters will be less costly as compared to closed feedwater heaters. Open feedwater heaters will also be simple in design and with high capacity of heat transfer.

Closed feedwater heaters

In case of closed feedwater heaters, extracted steam or bled steam will not be mixed with feedwater. As we know that feedwater heater will be one type of heat exchanger of shell and tube type and therefore feedwater will flow through the tubes of heat exchanger and bled steam will flow within the shell over the surface of the tubes of heat exchanger.

Bled steam will be condensed over the surface of tubes and heat released during the process of condensation of bled steam will be transferred to the feedwater and hence feedwater will secure heat energy during flowing through the feedwater heater.

Following figure displayed here indicates the regenerative cycle block diagram with closed feedwater heaters.

Let us see here the basic concept of closed feedwater heater with the help of following image

Closed feedwater heaters will be costly as compared with open feedwater heaters.

Open feedwater heater versus closed feed waterheater

Open feedwater heater	Closed feedwater heater
Design will be simple	Design will be complex
Heat transfer capacity will be quite good as compared with closed feedwater heater	Heat transfer capacity will be less effective as compared with open feedwater heater
Open feedwater heater will be less costly	Closed feedwater heater will be more costly
Open feedwater heaters will require large area	Closed feedwater heaters will require less area
Open feedwater heaters will require a pump for each heater for handling the feedwater flow	Closed feedwater heaters will require only a single pump for the main feedwater flow regardless with the number of heaters

Do you have any suggestions? Please write in comment box.

We will discuss another article in our next post i.e. " Rankine cycle with reheat and regeneration" 

References

Engineering thermodynamics by P. K. Nag

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