HVAC: Calculating enthelpy change after humidification

[EVriderDK]

- Without chart!

I know I can bring up my molliers chart and use that enthalpy scale and draw a line and all that, but I want to know how to calculate it, and I cannot find any formula.

Right now I have a problem where I only have the absolute moisture content before the humidifyer at w=0.0057 kg/kg and I know the flowrate and the enthalpy of the steam, and also all the data after the humidifyer, so now I need to know the enthalpy before the humidifyer. How to do?

Thanks in advance.

 

[Chestermiller]

You want to calculate the enthalpy of a stream containing air and water vapor. Suppose you had pure air and pure water vapor at the same temperature and pressure as their temperature and partial pressures in the mixture. Could you determine the enthalpy per unit mass of each of these pure species?

Chet

 

[EVriderDK]

I'm not sure about what you mean, but for calculating enthalpy of air and water I guess these formulas are relevant: h.dry air=(c.p.air⋅t.dry air) and for water it is h.water=(c.p.water * t.water + h.heat of evaporization)

 

[Chestermiller]

I'm not sure about what you mean, but for calculating enthalpy of air and water I guess these formulas are relevant: h.dry air=(c.p.air⋅t.dry air) and for water it is h.water=(c.p.water * t.water + h.heat of evaporization)

This is correct if you are referencing to 0C, and you are choosing the reference point for water as liquid at 0 C. Also, if you want to be more precise, you need to integrate Cp dT, and you need to specify heat of vaporization at 0C. It also assumes c.p.water refers to the heat capacity of water vapor. Is this all what you have in mind? Since, in the mixture, the air and water vapor are at the same temperature, you don't need to specify t.dry air and t.water

Once you have the enthalpies per unit mass of the pure species, the enthalpy per unit mass of the mixture is equal to sum of the enthalpy of each pure gas multiplied by its mass fraction. This assumes ideal gas and solution behavior.

Chet

 

[EVriderDK]

I am not sure I get it.

I have a heater, and after the heater there is a humidifier. I do know all properties before the heater and also after the humidifier, but I am interested in the properties between the heater and the humidifier. I have flowrate and enthalpy of the steam from the humidifier, so I can find the humidity ratio before the humidifier. Then I can use the scale on the mollier chart and find the intersection. But what if I don't have a chart and I want to find the properties before or after the humidifier?

 

[Chestermiller]

I am not sure I get it.

I have a heater, and after the heater there is a humidifier. I do know all properties before the heater and also after the humidifier, but I am interested in the properties between the heater and the humidifier. I have flowrate and enthalpy of the steam from the humidifier, so I can find the humidity ratio before the humidifier. Then I can use the scale on the mollier chart and find the intersection. But what if I don't have a chart and I want to find the properties before or after the humidifier?

I don't quite follow. Can you provide the details for the specific situation?

Chet

 

[EVriderDK]

Hello again.

Yes you can see a diagram here: http://peecee.dk/upload/view/445952

 

[Chestermiller]

Hello again.

Yes you can see a diagram here: http://peecee.dk/upload/view/445952

It looks like you are interested in the characteristics of stream 3, correct? Do you know the temperature of stream 3? It looks like you know the characteristics of stream 2, correct?

Chet

 

[EVriderDK]

Yes, I know stream 2, but only water content of stream 3, so I need one more property.

I also know stream 4 so I guess I should be able to take the enthalpy from stream 4 and subtract the energy that is added from the water, and then I should be able to get the enthalpy before the humidifier. - Just like I do to get the water content: x3=x4-(18kg/h / 6000kg/h) But I don't know how.

Picture in full size: http://peecee.dk/upload/view/445952/full Sorry.

 

[Chestermiller]

Yes, I know stream 2, but only water content of stream 3, so I need one more property.

I also know stream 4 so I guess I should be able to take the enthalpy from stream 4 and subtract the energy that is added from the water, and then I should be able to get the enthalpy before the humidifier. - Just like I do to get the water content: x3=x4-(18kg/h / 6000kg/h) But I don't know how.

Picture in full size: http://peecee.dk/upload/view/445952/full Sorry.

You know streams 2 and 4, and you want to determine stream 3, correct? You also have that steam stream coming into join stream 3 to form stream 4.

Tell me what you know about streams 2 and 4:

1. Mass flow rate
2. Temperature
3. Specific or relative humidity
4. Enthalpy per unit mass

What is the reference of temperature and pressure for zero enthalpy of (a) pure water and (b) dry air?

Chet