Overall heat transfer coeficcient fo ammonia heat exchanger

[luissandoval]

Hi guys:

I need to compute a coil for ammonia Heat exchanger

and i have the following dates:

Q = 400000 Kcal/h
T cold fluid = -22ºC (NH3)
T Hot fluid = -15 º C ( Brine sodium chloride)

IF i use

Q = (Tc - Th) x U X A

For calculate "A"

I need to Know "U" the overall heat transfer coefficient but The determination of U is often tedious and needs data not yet available in preliminary stages of the design somebody has typical values of U for quickly estimating the required surface area. I hope that somone can help me.

PD: This coil works in a flooded system.

Best regards.


Luis

[Dacosta]

I just did a review class for my fellow engineers on this subject.

Here are my class notes

Calculating Heat Exchanger Capacity

The basic equations for calculating capacity is the energy balance equation

Q=M (hin-hout) or Q = M Cp (Tin-Tout)

Where
Q = heat transfer (BTU/hr)
M= mass flow rate (lb/hr)
h = Enthalpy (btu/lb)
Cp = Specific heat (Btu/lb-ºF)
T = Temperature (ºF)

For refrigerants their thermodynamic tables it is easier to use the first equation, for water and other fluids such as brines and glycol it will be easier to use the second equation.


Assuming minimal loss to the heat exchanger itself and to the surrounding the energy balance equations must balance out.

Mrefrigerant (hin-hout) = Mwater Cp (Tin-Tout)

Once the flow rates have been determined the required size of the heat exchanger can be determined.


The standard way to calculate the size based on capacity of a heat exchanger is the Log Mean Temperature Difference (LMTD)

And is given by this equation

Q= U * A* (ΔTa- ΔTb)/ ln(ΔTa/ ΔTb)

Where
Q= heat transfer (BTU/hr)
U = Heat transfer coefficient (BTU/hr-ft2-ºF)
A= heat transfer area (ft2)
ΔTa = Temperature difference on the “A” side
ΔTb = Temperature difference on “B” side

However the value of the Heat Transfer coefficient is experimentally determined, and therefore changes with different fluids. In some cases the exit temperatures is unknown, in these cases it may be easier to start with the limitations of in incoming temperatures and the size of the heat exchanger first.

This method is the Number of Transfer Units (NTU) method

Qmax = Cmin (Thot,in – Tcold,in)

Where
Qmax = the theoretical maximum heat transfer rate (BTU/hr)
Cmin = the smaller of the two specific heats (BTU/lb-ºF)
Thot,in = The hot incoming temperature (ºF)
Tcold,in = The cold incoming temperature (ºF)

We can define an Effectiveness ratio as

E= Qacutal/Qmax


E can be calculated based on thermodynamic principles and tables are available to calculate E based on different designs of heat exchangers. However for system that has either vaporization or condensation, E can be simplified by the following equation.

E = 1 – e-NTU

Where
NTU = UA/Cmin
U = Heat transfer coefficient (BTU/hr-ft2-ºF)
A= heat transfer area (ft2)
Cmin = the smaller of the two specific heats (BTU/lb-ºF)


If an initial conditions is known, E can be calculated, and the heat transfer coefficient and heat transfer area can also be calculated. These values can be utilized in off conditions or in similar applications.



U value for a calcuim or sodium brine 80-125 btu/hr-ft2-ºF

(Mechanical Engineering Reference Manual for the PE exam, Lindeburg)

[Josip]

Hi guys:

I need to compute a coil for ammonia Heat exchanger

and i have the following dates:

Q = 400000 Kcal/h
T cold fluid = -22ºC (NH3)
T Hot fluid = -15 º C ( Brine sodium chloride)

IF i use

Q = (Tc - Th) x U X A

For calculate "A"

I need to Know "U" the overall heat transfer coefficient but The determination of U is often tedious and needs data not yet available in preliminary stages of the design somebody has typical values of U for quickly estimating the required surface area. I hope that somone can help me.

PD: This coil works in a flooded system.

Best regards.


Luis

SI units....



I just did a review class for my fellow engineers on this subject.

Here are my class notes

Calculating Heat Exchanger Capacity

The basic equations for calculating capacity is the energy balance equation

Q=M (hin-hout) or Q = M Cp (Tin-Tout)

Where
Q = heat transfer (BTU/hr)
M= mass flow rate (lb/hr)
h = Enthalpy (btu/lb)
Cp = Specific heat (Btu/lb-ºF)
T = Temperature (ºF)

For refrigerants their thermodynamic tables it is easier to use the first equation, for water and other fluids such as brines and glycol it will be easier to use the second equation.


Assuming minimal loss to the heat exchanger itself and to the surrounding the energy balance equations must balance out.

Mrefrigerant (hin-hout) = Mwater Cp (Tin-Tout)

Once the flow rates have been determined the required size of the heat exchanger can be determined.


The standard way to calculate the size based on capacity of a heat exchanger is the Log Mean Temperature Difference (LMTD)

And is given by this equation

Q= U * A* (ΔTa- ΔTb)/ ln(ΔTa/ ΔTb)

Where
Q= heat transfer (BTU/hr)
U = Heat transfer coefficient (BTU/hr-ft2-ºF)
A= heat transfer area (ft2)
ΔTa = Temperature difference on the “A” side
ΔTb = Temperature difference on “B” side

However the value of the Heat Transfer coefficient is experimentally determined, and therefore changes with different fluids. In some cases the exit temperatures is unknown, in these cases it may be easier to start with the limitations of in incoming temperatures and the size of the heat exchanger first.

This method is the Number of Transfer Units (NTU) method

Qmax = Cmin (Thot,in – Tcold,in)

Where
Qmax = the theoretical maximum heat transfer rate (BTU/hr)
Cmin = the smaller of the two specific heats (BTU/lb-ºF)
Thot,in = The hot incoming temperature (ºF)
Tcold,in = The cold incoming temperature (ºF)

We can define an Effectiveness ratio as

E= Qacutal/Qmax


E can be calculated based on thermodynamic principles and tables are available to calculate E based on different designs of heat exchangers. However for system that has either vaporization or condensation, E can be simplified by the following equation.

E = 1 – e-NTU

Where
NTU = UA/Cmin
U = Heat transfer coefficient (BTU/hr-ft2-ºF)
A= heat transfer area (ft2)
Cmin = the smaller of the two specific heats (BTU/lb-ºF)


If an initial conditions is known, E can be calculated, and the heat transfer coefficient and heat transfer area can also be calculated. These values can be utilized in off conditions or in similar applications.



U value for a calcuim or sodium brine 80-125 btu/hr-ft2-ºF

(Mechanical Engineering Reference Manual for the PE exam, Lindeburg)


IP units....

Hope not to late;)

Nice help Dacosta, thanks ... Luis take care about unit for U value for brine ;)

Best regards, Josip :)