Mismatched AHU and CDU for DX system

[HVAC68]

I have a direct expansion (DX) system, using R22 refrigerant. The AHU has been designed for 300KW and the air-cooled condensing unit has been designed for 214KW - reasons - probably, oversight ? I don't know.

These capacities are based on a saturated suction temperature of 6 deg C.

What would be the effect of having a mis-matched system like this ?

HVAC68

 

[quark]

Higher system pressure, motor overloading and subsequent burnout. Refrigerating effect and COP of the system decreases.

 

[gepman]

You need to check the reasons for the mismatch. If the load at the AHU is not actually greater than the condensing unit capacity then you could change the TEV and limit the maximum R22 flow to the evaporator to the capacity of the compressor and condenser, then you just have an oversized evaporator coil and it would be more efficient.

You actually can't evaporate more at the AHU than the compressor at the CU will pull, so your suction pressure will rise. With the rise in suction pressure will come an increase in mass flow capacity of the compressor. The difference in enthalpy between saturated vapor and saturated liquid will decrease with this rise in suction pressure but it will not decrease as fast the increase in mass flow capacity (due to the increase in density of the gas). Therefore as the suction pressure increases your refrigeration capacity increases putting more load on the condenser and increasing the discharge pressure. This higher discharge pressure will cause problems as quark stated and the system should trip on high head pressure or motor overload depending on the particular capacities of the compressor, condenser, trip setting, and evaporator load.

Therefore I guess I would change out the TEV in any case (whether or not the actual evaporator load is greater than the CU capacity) in order to avoid any potential problems.

 

[imok2]

Air conditioning systems are several components designed to work together in order to provide the best possible cooling for an application. So why would anyone consider replacing one coil and not the other? The answer isn’t that simple, and the 13 SEER transition has brought the ongoing mismatched coil debate to a head. At the heart of the debate is what should contractors do if they replace an existing, lower-efficiency condensing unit with a 13 SEER unit. Should the existing outdoor coil automatically be replaced as well? Or can you simply install a thermal expansion valve (TXV) on the existing indoor coil to increase its efficiency? The answer is no! Mismatching results in a totally defective sensible heat ratio and this normally leads to uncomfortable conditions more times then not. It’s a mess!
A system that is properly matched will operate at its fullest efficiency, providing you with a more comfortable environment at a lower operating cost. If your system is not properly matched, you will be paying more than what you are getting and your electric bills will most likely be a lot higher. In addition, if a system is installed that will result in an SEER below the federal minimum of 13 then if it’s not an actual violation of the law, it is certainly against the spirit of the federal minimum energy efficiency standards passed by Congress in order to save energy for the benefit of the nation and the consumer.

 

[AbbyNormal]

it will most likely balance out at a higher SST than 6C maybe closer to 9 or 10C.

The AHU capapcity will drop from what it is rated at, the CU capacity will increase

SHR of evap coil will increase

Plot AHU capacity at a few SST, plot CU capacity, see where the lines intersect. Be in the ballpark. Would have to allow for suction line pressure loss if you wantred to be more exact.


wild ass guess 257 kW, just averaging them



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

I would bet COP increases

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[TXiceman]

You need to do a rebalance of the AHU and CU. It is not hard to do. It may have been done for a reason. You will have a higher suction pressure, higher discharge pressure and a higher leaving air temp which results in less moisture being removed from the air.

Ken

Ken
KE5DFR

 

[AbbyNormal]

Evap coil capacity decreases as suction pressure (and therefore refrigerant temperature)increases.

The compressor is opposite, its capacity increases as suction pressure increases.

A big part of the battle with 'high seer' is to get more work out of a smaller compressor and the easiest way to do that is to get the suction pressure up, as well as make the condenser larger to make it get rid of heat easier.

Most condensers should have capacities listed against a range of suction pressures/temperatures, probably from 2C to 10C, at ambient temps ranging from about 30C to 45C.

So you plot your condensing unit capacity vs suction temp for the design ambient. On the same chart plot how the evap coil capacity drops with suction temp. Where they intersect will give you an idea of what kind of capacity you will be getting. You will have some suction line losses to account for if you want to try to be more accurate.

Could have been a deliberate mismatch for a big sensible load like a server room, where you want a lot of air and temperature control.

Maybe its a dry , high altitude application.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[HVAC68]

Thanks for the posts. I couldn't respond earlier.

The system is a about to be installed !!! For some strange reason (don't ask me why !) it has been mismatched. The application is for an assembly area with a high latent load.

The expansion valve is yet to be sized or ordered. I have got into this mid-way and don't have enough legacy information. The project was shelved for sometime, after the equipment was delivered. We have recalculated the space load and it looks like closer to the condensing unit capacity.

Thanks again for the response. Will try and match the "mismatched" units !

HVAC68

 

[sterl]

Abby said it: This a normal selection for a high sensible load on recirculation only, for a space like server room or switching room....For an assembly area or similar, want to run low suction and deep circuited coil. Only other reason to mismatch AC units such that Evap bigger than CU is to maintain higher RH such as for certain types of storages or process spaces (vegetables, rind fruits, flowers, nuts, cheeses, certain but not all baked goods) and only select ones of those are going to operate with sat suction near 38...

Maybe a stupid question, but is the Evap Coil split into 3? That is: Are there 3-distributors, 3-fan sets and so on?

If all this on a penthouse or central trunk,could be somebod put in enough Low Side for the someday but only enough High Side for Today?

 

[HVAC68]

No. This seems to be a mistake at the time of design, that's it. The application for a place of assembly (Mosque) - very high latent load. The evap. coil is split into 2 - and is so the condensing unit - it's a twin-circuit condensing unit.

HVAC68

 

[PatBethea]

I am finishing up a church this afternoon that I designed with "mismatched" AHU and CU. In my particular case, I'm cooling a Fellowship Hall with two 10-ton air handlers. The air handlers have dual refrigeration circuits that are going to a pair of 4-ton condensing units. When I first looked at it, I was planning on two 7.5-ton (matched) units. However, my air flow requirements were near the limits of the air handlers I selected - I would have had to keep the ESP under 0.4" to achieve the correct air flow. Unfortunately, the distance from my mechanical room to the Fellowship Hall is 200' - keeping below 0.4" didn't seem feasible. So, I jumped up to a 10-ton AHU and hooked the two 4-ton condensing units to it. This will meet my cooling capacity requirements and my air flow requirements.

 

[AbbyNormal]

can usually order over sized blower motors

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[imok2]

From what you said about a high latent load this seems to be the worse type of application you could use as was mentioned several times the sensible heat factor will (should) increase when you will need greater latent removal
Good luck

 

[imok2]

In order to achieve high SEER ratings, condensers and evaporators are generally btu/hr oversized and compressors are btu/hr undersized. Many 12 SEER 18,000 btu/hr condensers have 16,500 btu/hr compressors and 2 ton evaporator coils. This combination could make reducing humidity more difficult. The SEER increase is based on higher evaporator temperatures and suction line pressures for increased volumetric capacity, permitting the use of lower amp draw compressors. Manufacturer's should design units with a little more compressor btu/hr capacity for very humid climate regions, so TEV refrigerant temperature controlled evaporator coils could be set closer to the freezing point.
The above is kinda like your situation where you have a large evaporator and a small condenser. Notice what it says about the humidity