High Pressure Economizer (HPEC)
If you are majored in economics, you probably wonder what an Economizer is within a power plant. Here is an article that explains why we save money by using economizer for HRSG (Heat Recovery Steam Generator)
Why Do I Need Economizer?
Economizer preheats the water to near saturated temperature so that evaporator can then turn the water into steam in a more efficient way. It is certainly not a requirement to preheat the water, but it does take longer if you dump cold water directly into the evaporator. If you have a lot of heat source and you don't care about how long it takes for water to boil, then you can probably get rid of economizer. But we live in the society where time is precious and we want the water to boil instantaneously. So here are the two main reasons why economizer is needed: The heat source flue gas coming from the gas turbine exhaust is fixed, so we want to utilize it in the best way possible. The flue gas going into the HRSG is at 1050F and comes out about 140F. So that is a lot heat transfer and economizer plays a major role during this process. It is like you want to squeeze every bit of energy out so you don't waste it. So yeah, make economic sense huh? The time required to generate steam is continuous, you don't want to have any settling time for water to rise to saturation temperature. So economizer helps the evaporator to generate steam continuously in a steady state fashion. So now you see how economizer saves the energy (equals to money) as well as time.
The amount of heat needed for water to evaporate is different. It takes longer to boil cold water versus hot water at atmosphere pressure (14.7 PSIG). It is the same principle with high pressure boiler - hotter the water you dump into the evaporator, faster it can generate steam. This temperature difference between the economizer outlet to the saturated temperature inside the drum is called approach point. It is usually kept less than 5 degrees. Although, you don't want to keep to too close where steaming in the economizer can happen. The economizer outlet temperature is determined by the amount of heat transfer in the economizer absorbing heat from the flue gas, it also depends on the how much the water flow going through the economizer. For example, with same amount of heat input, with large amount of water flowing through the economizer, your economizer outlet temperature will not get hot simply because you have to heat up so much water. On the other hand, with same amount of heat input, with very small amount of water through the economizer, you will get the outlet temperature really hot. So the question now is how do you control the amount of water flowing through the economizer? The flow rate of generated steam depends on the amount of heat available in the flue gas as well as the back pressure from the steam turbine. In the commissioning phase, especially the steam blow process, the back pressure is low and the steam generation is high. Therefore a lot of water will be flowing through the HP Economizer. With all being said above, don't forget about other systems, like the low pressure economizer, which has very slow water flow, therefore you will see steaming in the LPEC pretty often during the steam blow.
Lower steam flow rate in the superheater means lower water flow rate in the economizer. When you have a lot of heat transfer in the economizer and the water flowing through it is low, you can have steaming inside the economizer. This will cause some upset conditions such as: The steaming occurs and the pressure build up in the top crossover of the economizer and then if you open the vent, the pressure will spike into the drum and cause the drum level surge which trips the gas turbine. The steaming occurs and the pressure bottled up inside the economizer, once it gets higher than the drum pressure, it is hard to flow water into the drum because reverse flow might happen and eventually lift the safety valve. Steaming inside the economizer does happen very often during steam blow. I think this is the fun part of commissioning power plant. You need to figure out all sorts of odd conditions that won't be seen in a normal operation. That is why understanding the thermodynamics as well as the control logic makes you feel confident when facing issues like this.
Economizer bypass is used to cool the water down by bypassing economizer modules. The bypass works like a attemperator where you mix the cold feedwater with hot economizer outlet water to make the temperature cooler before it goes into the drum. What if the temperature in the economizer outlet is still too hot but you are already spraying it down with 100% bypass open? This can happen and your only solution is to increase the steam generation so that more water can flows through the economizer. It is possible to let other systems remove heat from the flue gas, but that can be little complex and hard to implement. Another example, in a low flow condition for the LPEC (Low Pressure Economizer), more water bypassing the economizer and less water flowing through the economizer, with more heat absorption, you actually get steaming in the LPEC. So with more bypass and lower economizer flow, you actually increasing the economizer outlet temperature. You might ask isn't that counter acting with the purpose of bypass temperature control valve? Yes, you are correct, this is exactly why thermal process is too tricky to figure out sometimes. I would call my thermal engineers then ask them why.
Single versus Multiple Passes
Economizer module doesn't have to be single pass like those superheater modules. The water goes into the module and circulates up and down several times before it exits the module. I was confused in the beginning about how come water goes in at the bottom and then comes out at the bottom. This is because the header is actually separated to create multiple passes. This creates more heat transfer because the same chunk of water sees the flue gas longer.
In order to get rid of non-condensable gases in the economizer crossover, you can use the economizer vent which is a motor operated valve and that will help the steaming inside the economizer during steam blow. Make sure you don't have this valve cycle because it will make the drum level very erratic.
Here are several control system logics and concepts you need to understand in terms of economizer How to control the approach point using the economizer bypass control valve. How does back pressure from steam turbine affects the steam generation How does the economizer vent works and what impacts does it have when cycling How to identify steaming inside the economizer.
I hope this article convinces you how important the economizer is given that not much control logics are associated with this section of the HRSG. I personally experienced the economizer steaming being the most common issue during the steam blow, so make sure you can understand the thermodynamics behind it. The level control valve can be on the economizer outlet or on the inlet side, which it is called "closable economizer". This is another topic to discuss because it affects how the approach point and the bypass control valve works together.