A question about suction line heat exchanger

Hello all,

I have a question about suction line heat exchanger. I learned from some papers that in R134a, R404A and R600 systems, suction line heat exchanger is very helpful to increase energy efficiency; but it does not work well in R22 systems. Could anyone explain why this happens?

In my opinion, as suction line heat exchanger always increases system super heat and reduce the refrigerant mass flow, adding suction line heat exchanger always reduce system cooling capacity. Is it correct?

How about R410a? Does the suction line heat exchanger help to indrease the energy efficiency?

Thanks
ywz

I had one paper on this. I shall try & find out why it is not so efficient with R 22. If I get it I shall port it on this link. But you are right that it is not so efficient for R 22.

http://www.irc.wisc.edu/file.php?id=49

I got it. Have look at this paper.

Thanks smpsmp45. The paper gives a very clear explanation. I got it.

Ywz.
If you or someone else got it can you explain to me in layman's terms (plain / simple english..u.k. terminology)
Because I looked at it and it frightened me. To many big words and numbers for my liking.
So if anyone would take pity on me I would be greatfull.
Grizzly

Hello Grizzly,

Here is a short explanation:

The liquid suction heat exchanger affects the performance of a refrigeration system in the following ways:

• It reduces the liquid refrigerant enthalpy and increases the enthalpy difference in the evaporator.
• It increases the temperature and specific volume of the refrigerant at compressor suction port and reduces the mass flow in the system.

Cool capacity = enthalpy difference X mass flow

The first factor has positive influence to the cooling capacity; the second factor has negative influence to the cooling capacity. When we talk about the influence of a suction line heat exchanger, we need to figure out which factor changes much.

For some refrigerants, such as R404a and R134a, the influence of enthalpy difference increasing (factor 1) is greater than the influence of mass flow reducing (factor 2). So the suction line heat exchanger increases the cooling capacity. For some other refrigerants, such as R32 and R717, the influence of mass flow reducing (factor 2) is greater than the influence of enthalpy difference increasing (factor 1). So the suction line heat exchanger reduces the cooling capacity.

Dear YWZ,

Oh! your explaination is really good for anyone to understand!!

YWZ.
I am impressed! Thank you
Grizzlyhttp://www.refrigeration-engineer.co...ons/icon14.gif

yes, it may reduce the overall cooling cap.
But what if i have accounted in the compressor selection, then the issue of reducing cap does not exists!

This is similar to oil injection cooling and external oil cooling for oil cooler.

ah,That's so good,I just to find some way to increase the capcity and COP in designing R134a system,could you or anyone suggest for liquid-suction heat exchanger designing or some notes?

If you want to increase the COP of your system, increase your LP and decrease your HP. That will give you huge
savings compared to a Suction heat exchanger.

All the other things aren't worth the money,... that's my opinion.

Are you designing a small system with TEV or capillary?

may be this helps to understand the use of a suction heater.
a compressor is design for presserise dry gas. incase we deliver little droplets of refrigerant into the suction of the compressor, the compression heat makes first gass of the droplets. This cost energy en let the compressor not work efffecient. if we over heat the droplets before the droplets enters the suction of the compressor the compressor can do what he is design for.

the energy , which we need to do this we take form the system.

Some refrigerants work in a glide, that mean that all parts not at the same time comming into gas. In this cases a for heating of suction heating is welcome. the refrigarant R404A is well know about this( build up of several items),in the flied we measure a overheating, but are all parts into gas? unfortunally not.

R22 shown not this glide,therfore we can not win much in energy. I hope this makes things a bit more clear.

Best regards

Martin Wierbosch [Moderator Edit: Email Removed]

Hi there,

The problem is not accounted for the cooling capacity only the main concern is the system COP. What YWZ has mentioned is correct but there is some is missing which is

COP = Evaporator enthalpy difference "refrigerating effect" / Compressor enthalpy difference "Compressor work"

And the mass flow has no rule here to be involved in this equation. The liquid suction heat exchanger has two adverse effects:

1- It reduces the liquid refrigerant enthalpy and increases the enthalpy difference in the evaporator. (as mentioned by YWZ)

2- it causes an additional pressure drop in the suction line prior to enter the compressor and hence reduces also the refrigerant enthaply entering the compressor and in turn to increase compressor work input.

So two competiting effects at work and according to which effect is the dominator the liquid suction heat exchanger (HE) comply either assist or harm the system performance.

So it may be concluded as the finding in this paper if the refrigerant has low liquid specific heat the opportunity for the HE to assist the system is high. The reason is the lower specific heat for the liquid the more temperture difference for the subcooling effect in the HE and it can prevail the drop in the refrigerant pressure and it is satisfied with R134a and R410A not like R22 which has higher liquid sp. heat.

cheers

In addition this topic has been discussed in many threads like this:

http://www.refrigeration-engineer.co...read.php?t=859

Thank you Mohamed Khamis for the thread.

Quote:

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Originally Posted by Samarjit Sen

Thank you Mohamed Khamis for the thread.

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Hi Samarjit, Very glad to see u again here in this froum

All best regards

Mohamed