Condensers in series 1

[llf12]

I need to install two condensers in series at my R134a heat recovery chiller, one is heat reclaim condenser and the other is evaporative condenser. Condensation may occur simultaneously at 1st condenser or 2nd condenser. Gas, liquid or both may exist at piping between condensers.Does any one know how to tackle the liquid and oil problems?

 

[pennpoint]

llf12

You said series connected but I just want to be sure we are on the same page.
Series or parallel?

pennpoint

 

[imok2]

llf12
Oil separators can effectively remove oil from refrigerant vapors. The oil removed usually collects at the bottom of the separator. From there, it is returned periodically to the crankcase. The relationships of solubility, pressure, and temperature between oil and refrigerant in the crankcase also apply here. Too much refrigerant can dissolve in oil and present a real problem when oil gets to the crankcase. The separator should be warm enough to keep oil at a higher temperature than the refrigerant in the condenser. For this reason, the oil separator should be close to the compressor. It might be good to insulate or heat the lower part of the separator. Also you want to keep the refrigerant velocity at 1500 ft/sec on vertical risers and 750ft/sec on horizontal runs.

 

[lGerryl]

There are several issues to contend with here. Oil return is not usually a concern, as the oil is miscible with the liquid refrigerant and if the liquid refrigerant flows correctly so will the oil.
The issues are with refrigerant managment. You will need a receiver, and some method of flooding head pressure control.

Series condensers, both of which are sized for 100% condensing, have situations where ... if the first condenser is utilized at maximum capacity, then liquid will leave that condenser, and the second condenser in series will have to be liquid full. Using a flooding head pressure control system and a correctly sized reciever will allow this type of system to function effectively. The trick is to design the head pressure control to handle the pressure drop through the first condenser when reheat is not required, and yet have sufficient pressure drop, when reheat is required, to load the circuits correctly.

Also of concern, is the temperature of the first coil when reheat is not required. The superheated discharge gas can easily exceed 200°F when reheat is not required. All components close to this coil must be designed to handle this temperature. Such as damper motors, bushings etc.

Have faith, I have designed several systems of this type, and have had very good success.

Gerry

PS to IMOK2
I belive that you mean 1500 ft/min. rather than feet per second.
1500 feet per second exceeds mach one.

 

[llf12]

Gerry,

Thank!

Is there any device to seperate the liquid and gas from 1st condenser? So that any liquid will bypass the second condenser and directly go to receiver.

I also concern about the condensing temperature control. The unit will be operated at much higher temperature whenever 100% condensation occur at 1st condenser (assume to be shell and tube type), and will be much lower when no condensation at 1st condenser(heat rejection thru desuperheat at 1st condenser and evaporative condenser). How about the condensing temperature in between?
Is it possible to use speed control of the recirculation pump and fan to maintain the hot water outlet temperature(around 50 deg C and is constant flow)?

 

[imok2]

Gerry , thanks for the correction. Now I was wondering why couldn't illf12 parallel his condensers (1) shell & tube and (1) evap cond. Say come off his compressor with the discharge line and "Y" either into S & T os EC. "Y" out to a liq. receiver.Control a set of valves (1) for each condenser, Could use Pneumatic controller or electronic with opposite control on the valves with sensor in control medium. What do you think?

 

[lGerryl]

llf12

A simple high side float can separate liquid from the reheat flow and bypass the secondary condenser. However, this is best suited for a parallel style system, and whereas this can be extremely simple solution where you only need a small portion of the total heat of rejection,(10 to 20%) it becomes very dificult to effectivly control when the heat rejection is greater than that.

The trick to the whole thing, is that there must be enough condensing surface somewhere to provide 100% condensing. No more... No less. Q=UA¦T
where Q is the heat rejected
U is the heat transfer coeficient of the heat exchanger,
A is the effective surface area
¦T is the difference between SDT and ambient (or liquid)
(this is simplified quite a bit but shows the overall relationship)

Therefore, if you want a higher condensing temperature, there must be less condensing area, or warmer condensing medium or both.
Most valve manufacturors, (Sporlan, ALco, Parker, Danfos etc) will show methods of doing this in their catalogues and which parts are required to achieve the desired results.

hope this helps.

Imok2

With a sufficiently intelegent control system this could work.
But the real secrets to these systems are
Q "where is the liquid refrigerant going to go"
A always to the colder spot

Q "where will the heat be rejected to?
A The heat will try to go to the coldest spot.

Q "how do I get the heat to go where I want it too.
A make the other condenser have a warmer temperature (cycle fans, close dampers, or smaller EFFECTIVE AREA, flood the tubes with liquid refrigerant, etc)

The secret to success is to know where your refrigerant chrg is going to go.

 

[sterl]

Parker publishes a very good manual on Heat Reclaim, Bulletin HR, Refrigerating Specialties Division in Broadview, IL. Its older and focused on the Mineral Oil Halocarbons, but the principles apply to the more modern halocarbons as well...Course if its ammonia or CO2 or other "natural" refrigerant, the game is considerably different. What refrgerant we dealing with?

If the evaporative condenser is an indication of the extent of the system, and if you are subject to low ambient temperatures, backflooding the ambient condenser may be cost prohibitive...

Depending on the nature and relative location of the HR condenser, and the relative capacities, and the turndown of the compressor system, the Heat Reclaim Arrangement will most likely become: Parallel on the Liquid Side, Series on the Hot Gas Side, Separator downstream of the HR condenser with HP float valve delivery to the HPR.

The means of compressor cooling is very much involved with this...If you are thermosyphon cooled, you will need to establish (2) HP liquid reservoirs either at different elevations or at different pressures with controlled liquid level in one of them...With liquid injection or flash economizers, the control scheme gets even more involved.

If you have any Hot Gas Defrost or other diversions of Hot Gas such as a transfer drum arrangement, you will definitely need to apply a regulator to the Main Discharge line or you won't have either pressure or liquid level when you need it most.

This starting to sound like fun yet?

 

[boyceg]

Slightly off topic, but may be helpful...
I assume you want to recover superheat energy. If so, consider a liquid line to suction line (LLSL) heat exchanger. This improves the COP of R134a systems and increases the superheat temperature and energy. The LLSL COP increase with R134a is especially effective for greater temperature lifts. LLSL heat exchange will increase the superheat temperature and energy of R22 systems, but does almost nothing for COP.
Boyceg