A/C Heat Exchanger between pressure & suction lines 1

[jamesv]

I'm an engineer still learning about A/C. Two texts I've read state that installing a heat exchanger to transfer heat from the hot line between the compressor and condenser, to the cold line between the expansion valve and evaporator, will increase system efficiency. (at a minimum, having these lines run parallel and in contact with each other would be beneficial)
An installation manual I recently read advises against this however. Any elaboration on this subject, particularly as it relates to a home A/C installation, would be appreciated.

 

[KenRad]

You wouldn't want to add heat between the expansion valve and the evaporator; this would reduce capacity. I think the "cold line" that you are talking about should be downstream of the evaporator.

I've never heard of this, and fail to see the benefit in this arrangement, but I'm no refrigeration expert.

 

[ClydeMule]

JamesV,

We use a liquid to suction line heat exchanger, this sounds like what you are describing. It is basically a simple shell and tube heat exchnager, some models are literally a 3/8" tube wrapped around a 12" length of 7/8" pipe.

The idea is that the hot liquid 100-120 F or so heats up the relatively cool suction gas @ 45-60 F or so. It superheats the gas and subcools or adds subcooling to the liquid.

Unfortunately may people incorrectly state that it adds system efficiency. In does in a sense, but the way it is ususally described makes it sound like you magically can increase the capacity of any refrigeration system. What really happens is that by subcooling the liquid you slightly increase evap capacity, but at the same time you superheat the gas, which reduces compressor capacity. There is really no gain. The beneift is that by increasing the subcooling you eliminate (reduce) the chance of liquid flashing to vapor before entering the TEV. It also affords some compressor protection by further boiling off liquid in the suction line.

But it is like taking tylenol or something, there really is no benefit unless something is wrong.

Check out Packless, Parker, or Refrigeration Research websites to see one.

Good Luck!

Clyde

 

[VEEKRISH]

Jamesv,
What you are desribing is inaccurate. What is generally done is to have a 'LIQUID-SUCTION" heat excahnge. This does not in any way improve system efficiency or capacity. Moreover, it is done in cases when you have a rather large length of liquid line. In such cases, it is advisable to provide the LIQUID-SUCTION Heat Exchanger. This helps in sub-cooling the hot liquid and prevent liquyid evaporating before it reaches the expansion valve (due to increased ength).

 

[boyceg]

Liquid line to suction line (LLSL) heat exchanger...

The benefit is a strong function of the refrigerant used. LLSL heat exchange helps little with R22 with regard to COP. It helps COP a lot with R134a, especially if the condenser-evaporator delta T is large. If compressor superheat is being used, the LLSL heat exchanger will increase the superheat temperature and energy available, especially with R134a. According to the Copeland compressor performance software, R22 and R134a systems have nearly equal COP performance when 100% effective LLSL heat exchange is used.

Make sure the LLSL heat exchanger does not introduce much suction pressure drop. Suction pressure drop kills the advantage quickly. This MAY be a good application for an all stainless steel flat plate heat exchanger. My concern for flat plate heat exchangers is the pressure cycle life.
Boyceg

 

[554]

Hi to all and a merry christmas.

Heat exchanging between sution line and liquid line is used
for this purpose:

1: Increasing the system performance

2: Subcooling liquid refrigerant to prevent flashgas formation
at inlets to expansion devices

3: Fully evaporating any residual liquid that may remain in
the liquid-suction prior to reaching the compressor(s).

List of refrigerants in % (system performance)

R507A 21% R404A 18% R600A 13% R290 10% R134A 8%

R12/R407C 6% R410A 3% ----- R22 -2% R32 -6% R717 -11%

This is what you can get at max. for a system with a very
low pressure drop.

We use HX for the Thermostatic Flow Controller witch replaces the thermal valve in the cooling system.

Here is the link to the heat exchanger PDF file.

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

Best regards
Carlo Hansen

 

[25362]

How does the LLSL compare with regular economizers, in which the liquid refrigerant is sub-cooled by part vaporizing it in a heat exchanger, where the produced vapors are fed to a "higher" pressure stage in the compression system, in particular with R-717 ?

 

[boyceg]

Thanks, 554.

The link you gave above:

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

is excellent.
Boyceg

 

[sterl]

Econmizers represent a means of splitting the expansion step into (at least) 2 stages such that some of the heat of the warm liquid is removed at a (relatively) higher suction pressure. The vapor generated during expansion is thereby divided into 2-streams; the larger proportion of the enthalpy change generates vapor that is compressed through a lower pressure ratio, and the minor proportion remaining still is compressed through the entire pressure ratio. Result is a true gain in efficiency...

In terms of the compression process: Liquid to Suction Heat Exchangers have the net effect of decreasing the mass flow through the evaporator and compressor while raising the superheat at the compressor...With Hermetic recips or similar where the motor is cooled by the suction gas the penalty is fairly low, but with an open drive, the increase in superheat represents an decrease in the density of the suction vapor as it enters the compression chamber, thus decreasing the capacity of the compressor in mass flow terms.
For a given swept volume of machine, the trade off in total circuit COP will be determined by the slope of the V/T curve at the actual compressor suction, as compared to the h/T curve for the liquid at or near the expansion valve.

There can be an accompanying rise in suction pressure due to the marginal improvement in the heat transfer at the evaporator, which for a complete circuit represents a coutermanding effect, but the realized benefit depends on the nature of the heat exchanger. Low heat flux, high mass flux exchangers tend to develop an appreciable performance improvement; the other end of the HEX spectrum is not going to gain much.