parellel condenser issue

[Lc_shi]

Here need two condenser in parellel to one liquid line to expansion valve. One is air cooler fin tube type ,one is evaporative condenser, is there needed one liquid tank to receive the flow from the two condensers? who know the details ,pls give me some advice.thx!

regards
LC-

[Dan]

Parallel condenser issues have been hot debates for more than half a century. ASHRAE and others suggest individual liquid line drop legs of at least 6 feet and liquid seals (traps) to compensate for the different pressure drops between the two condensers. They also suggest vapor equalizing line piping. A common tank is also suggested.

The problem these suggestions are trying avoid, is the phenomenon of one condenser completely filling up with liquid. It has never been clear to me whether the nuisance was lack of availability of liquid during cold weather, or if it was lack of condensing surface during warm weather. I would set the two condensers up as a split condenser, where you automatically engage and disengage condenser operation based on discharge pressure or ambient conditions. I believe the Sporlan website has literature that addresses split condenser control. Good luck lc!

[Peter_1]

...suggest individual liquid line drop legs of at least 6 feet and liquid seals (traps) to compensate for the different pressure drops between the two condensers. They also suggest vapor equalizing line piping. A common tank is also suggested.

The problem these suggestions are trying avoid, is the phenomenon of one condenser completely filling up with liquid.

This is for me the complete answer and that's the way we're doing it also.

[Dan]

This is for me the complete answer and that's the way we're doing it also.

Peter, is this to avoid low discharge pressures during cold weather, or to avoid high discharge pressures during hot weather?

[Cofreth]

Recommend to install a check valve at each condenser outlet to prevent liquid refrigerant migrating to the cooler condenser.

[US Iceman]

Parallel condensers is always a tricky subject to discuss.

The main problem is due to the different pressure losses of the condensers. Air-cooled condenser have a higher pressure loss than evaporative condensers.

The difference in the outlet pressures of each condenser can be significant. This presents a problem with the liquid draining into the common receiver. If the receiver is at a higher pressure, the liquid cannot drain from the condensers. The traps and elevation difference create a static head in the liquid drain line to equalize the outlet pressures in the drain lines to the pressure in the receiver.

The equalizing line is required to maintain the receiver pressure at discharge/condensing pressure. Note: In some systems the equalizing line can be large!

The traps create a seal in the individual drain lines from each condenser. The height required is dependent on the pressure difference between the outlet of the condenser and the receiver pressure.

The trap height required can also increase in the winter. 6 feet may be OK for one application at one time. This is a general recommendation Dan and should not be used for design purposes.

The location of the receiver (and the color) can also affect the equalizing line size. The capacity does also.

As I said before, this is a tricky issue. The solution lies in understanding the pressure gradients that are developed during operation of each condenser and the interaction with the common receiver.

[Peter_1]

Cofreth, installing a check valve only in the condenser outlet to prevent migration...?

Dan, we do this to avoid HP during summer.

I follow the recommendations of Stoecker in his book Industrial Refrigeration where he explain on some pages how to do this.

It's like US Iceman said, it's tricky but the problem is that you can't - at least I can't - calculate nor predict the pressure drop of different condensers under varying load conditions. So make the liquid seals long enough. We've done it already some times and never had problems with it.

Or you can install also seriously oversized and inclined liquid lines from condenser to receiver.

Geoff Alder from South Africa described it also very clear in his book with very simple examples.

[US Iceman]

The other problem is that the condensers will allow liquid refrigerant to build-up in the coils, if the piping is not correct. This can happen during both the summer and winter.

The HP in the summer is a problem since the warm air will only provide a certain amount of cooling. In the winter time, the cold air does provide additional cooling. This helps to prevent the HP problem seen during the summer.

If the receiver liquid level is changing rapidly (usually a fast increase in liquid level), a problem will exist due to the piping. When the liquid level increases in the condenser(s), the static head generates sufficient pressure to "push" the liquid out of the condensers into the receiver. This is where the sudden liquid level changes come from.

On evaporative condensers at full load in the summer, the pressure drop is about 1 psi. For ammonia, this requires about 4 feet of static head to generate 1 psi. For R-22, 1 psi of pressure requires about 2 feet of static head.

During the winter, the pressure drop can increase (greater condensing ability), so the liquid traps will require greater depth to work. BAC, or Evapco (I can't remember right now) state that the trap depth be increased X 2 for other operating conditions. So, if the 6 feet is a recommended value, 12 feet may be required for other operating conditions.

This problem can also be complicated by condensers at different elevations. If this occurs, the piping is very messy!

[TXiceman]

As already noted by US Iceman, paralleling two unlike condenser is not easily done and is a lot of guess work. One condenser will tend to stack liquid and starve the other. The pressure loss through each must be equalized and the way to do this is long liquid traps into a common header.

I personally would not recommend the application of an A/C condenser and an evaporative condenser. It is just plain poor design in my opinion.

Ken

[US Iceman]

LC,

Why do you need to parallel an air-cooled condenser and an evaporative condenser? This seems redundant and could create more problems???

Can you share an explanation please?

[Lc_shi]

My intention is to reclaim fraction of condensing heat for air reheating. It's evaporative condenser chiller.Maybe there are other better ways to do the like job-

thanks for you valuable comment
happy new year and Gong Xi Fa Cai(make a fortune)-

rgds
LC

[Peter_1]

Lc_Shi

Perhaps those scans of a technical book of Danfoss 'Automation of refrigeration plant with heat recovery' can help you.
We use the most basic setup on page 42, not expensive and it works well.

[US Iceman]

I don't consider the task a tricky one. I think it's very straight forward.

Once you understand all that is occurring it is simple and straight forward. The "tricky" part is being aware of the many contributing factors and their effect on the situation.

1) System mass flow
2) Receiver surface area, color, and operating liquid level
3) Differential pressures between condenser drain lines
4) Differential pressure between the main drain line and the receiver
5) Equalizing line size
6) Effect of winter & summer operation

This is one area that is generally poorly understood and misapplied. As a result, the problem of liquid stacking up in the condenser coils is very common.

...we need to somehow allow vapour generated within the receiver to escape without hindering condenser drainage...

This is one of the biggest problems. Vapor generation plus the vapor displacement due to system mass flow. Developing the total mass flow through the equalizing line and then sizing the equalizing line, so that the pressure loss is lower than the static head generated by the trap height.


There are several ways to deal with this problem.

One, use adequately sized piping and properly install it. What we are discussing now.

Two, use a sloped, over-sized drain line to allow gravity drainage through a single pipe. This is what Peter_1 is describing.

Three, use high-side float valves and drain to a low-pressure receiver (or a controlled pressure receiver). Similar to steam heating coils.

Any one of the three above will work.

[US Iceman]

One additional item I will add to the discussion is the use of evaporative condensers by themselves. I am working on a project now where the condenser capacity is sufficient for the "calculated" heat rejection requirements, but does not work.

Evaporative processes (condensers, towers, etc.) can have a tendency to create their own micro-climates around the equipment. Interference with building structures, prevailing winds, and installation locations can severely impact the heat rejection capability.

Moist air recirculation due to any unknown or unforeseen impediments or sources can increase the entering wet bulb temperature. This can substantially reduce the heat rejection performance of the equipment.

As the piping discussion presented, the minor details can make the difference. The more subtle the influence is, the more drastic the impact from those I've seen.

Details, details, details....

[Argus]

My intention is to reclaim fraction of condensing heat for air reheating. It's evaporative condenser chiller.Maybe there are other better ways to do the like job-

This sounds more like a de-super-heater to me.
________
Suzuki JR80

[US Iceman]

My intention is to reclaim fraction of condensing heat for air reheating

Hi LC,

If you are using the air-cooled heat exchanger for reheating air do you have sufficient heat available to do this?

The heat rejection from the compressor has two components; temperature and heat energy. What I call quality and quantity.

First you have the heat energy of the refrigerant (kJ/kg or the quantity) and the temperature of the discharge vapor (degrees C, what I call quality). If you can extract sufficient heat to warm the air stream up, then you can use the temperature to reach the desired final air temperature.

This is sort of like an air source heat pump. You can extract heat from the cold, outside air but the air temperature in the house can be quite cool. In other words, heat has been transferred but the temperature may be lower than desirable.

Does that make sense??

[PobodysNerfect]

One additional item I will add to the discussion is the use of evaporative condensers by themselves. I am working on a project now where the condenser capacity is sufficient for the "calculated" heat rejection requirements, but does not work.

US Icemann, you should also take a close look at the water treatment chemicals. Some chemicals might effect negatively the heat transfer, especially if the dosage is not correct.

Saludos,

Jan

[US Iceman]

US Iceman, you should also take a close look at the water treatment chemicals. Some chemicals might effect negatively the heat transfer, especially if the dosage is not correct.

Thanks Jan. I already have noted this with the client. This is one problem that they have been having, but should have under control shortly.

While this is one of the major considerations, the end result was the condensers were marginally selected for the design weather conditions and did not include any site factors.

Thanks for the constructive advice.

[star882]

The problem these suggestions are trying avoid, is the phenomenon of one condenser completely filling up with liquid.

Couldn't the plumbing be run such that liquid in the condensers will drain into the receiver by gravity?
If that's not possible, couldn't a pump be installed to pump the refrigerant into the receiver?

A solution that may work is an electric valve at the inlet and a check valve at the outlet, with another valve and cap tube to slowly drain liquid from the condenser not in use to the evaporator inlet. It could use a valve similar to the ones used in heat pumps, connecting the condenser in use to the compressor and the one not in use to the cap tube.

[US Iceman]

Couldn't the plumbing be run such that liquid in the condensers will drain into the receiver by gravity?

The plumbing is one of the main problems. To achieve gravity drainage you have to ensure a fully equalized condition to prevent differences in pressure between the condenser drains and the receiver.

I am not a big proponent of using "gadgets" or other methods in any refrigeration system. I prefer to use the KISS principle on any design. Additional control valves, pumps, etc. adds to the system complexity.

If the piping is done correctly, all of the other requirements are then not necessary.