Lowest Approach

[meg]

Hello everybody,

What is the lowest approach one can select for shell and tube chiller, PHE during DX operation and Why?
can we select the approach equivalant to flooded chillers?

Waiting for positive reply.

[US Iceman]

What is the lowest approach one can select for shell and tube chiller, PHE during DX operation and Why?

On evaporators using DX feed you need some superheat for the valve to control correctly and to protect the compressor.

On a chiller the approach temperature is the leaving water temperature minus the saturated evaporating temperature.

Let's look at an HVAC chiller operating at standard conditions:

Entering water temp. = 55F (12.7C)
Leaving water temp. = 45F (7.2C)
Saturated evaporating temp. = 35F (1.6C)

In the above operating conditions the approach temperature is 10 degrees (5.6). This becomes your greatest amount of temperature differential you have available for superheating the suction gas.

You cannot obtain higher evaporator superheat than you have available approach temperature.

Using a smaller approach temperature also decreases the LMTD, which increases the total heat transfer surface required also.

The limit on how low of an approach temperature you can use is determined by:

A) how much surface area you are willing to pay for,
B) the minimum required superheat to protect the compressor

Waiting for positive reply.

What other kind of reply were you expecting?

[winfred.dela]

Waiting for positive reply.

positive?

Maybe like: as low as +0.5C (or maybe lower) for flooded where we can have cocurrent flow.

or Maybe around +10C for countercurrent flow in DX.

Figures above are estimates and varies depending on the design and actual conditions.

[johnyfreon]

On evaporators using DX feed you need some superheat for the valve to control correctly and to protect the compressor.

On a chiller the approach temperature is the leaving water temperature minus the saturated evaporating temperature.

Let's look at an HVAC chiller operating at standard conditions:

Entering water temp. = 55F (12.7C)
Leaving water temp. = 45F (7.2C)
Saturated evaporating temp. = 35F (1.6C)

In the above operating conditions the approach temperature is 10 degrees (5.6). This becomes your greatest amount of temperature differential you have available for superheating the suction gas.

You cannot obtain higher evaporator superheat than you have available approach temperature.

Using a smaller approach temperature also decreases the LMTD, which increases the total heat transfer surface required also.

The limit on how low of an approach temperature you can use is determined by:

A) how much surface area you are willing to pay for,
B) the minimum required superheat to protect the compressor

[/font]

What other kind of reply were you expecting?

Its been many years since I put my hands on a chiller and I’m a bit confused by your answer, I used to plot the approach in order to figure how good of a job the condenser was doing rejecting heat, for example; a fouled condenser will result on a higher approach.
Could explain a bit why the approach dictates the superheat? I always thought it was more of a combination of expansion valve setting, refrigerant temperature and pressures and evaporator’s loads.
As you said the smallest approach will translate into a higher rate of heat transfer but you lost me on the decrease of the LMTD (Qt= U X A X F).

[US Iceman]

The LMTD is affected as the temperature change, but don't worry about that right now.

The heat exchangers (chillers) are designed for a specific water temp (IN & OUT), evaporating temp. & cooling duty. Let's forget the cooling duty for now also.

According to the initial design parameters you have three temperatures (Water In & Out, & evaporating temp).

The approach temperature is the water Out temp. minus the evaporating temp.

Since the TXV bulb is responding to the evaporator outlet temperature and the water outlet temperature is fixed by design, the temperature available for superheating the refrigerant vapor is also fixed.

Therefore, the maximum superheat you can acheive is based on the evaporating temperature + the approach temperature.

I don't recommend adjusting the TXV's as a first course of action as these get played with way too much.

All of this "academic" discussion is based on design guidelines. When the system is in operation it should be operating at those same design parameters.

As you said the smallest approach will translate into a higher rate of heat transfer but you lost me on the decrease of the LMTD (Qt= U X A X F).

That's not exactly what I said. As the LMTD decreases you need more heat transfer surface for the same amount of heat to be exchanged between the fluid streams. The heat transfer rate does not increase, the required surface area does. The heat transfer rate actually decreases since the surface area has increased.

I'm going from memory now and it's been some time since I did a lot of this work, but I think the above is correct.

Where's Andy P when you need him?

[Josip]

Hi, all

All of this "academic" discussion is based on design guidelines. When the system is in operation it should be operating at those same design parameters.

This is what might be


....but having so many variables involved in heat exchange, which ultimately determine what can be

Hope you agree with me

Best regards, Josip

[US Iceman]

Hi Josip,

I do agree with you.

The one thing I believe we need to be aware of is just what you said. First of all the system is designed for a set of conditions. Secondly, we have to ensure those conditions are met in operation.

If the system operates at a different set of conditions than what it was designed for, then of course we will see, or could see many strange things.

As part of the design process, we can try to make sure it will operate correctly, or like in this case, if the approach temperature is selected too small, then something will happen.

Then again if it was a flooded chiller with a small approach temp. the chiller would be very large and cost more.