Mismatched AHU and CDU for DX system
Mismatched AHU and CDU for DX system
(OP)
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 ?
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





RE: Mismatched AHU and CDU for DX system
RE: Mismatched AHU and CDU for DX system
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.
RE: Mismatched AHU and CDU for DX system
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.
RE: Mismatched AHU and CDU for DX system
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.
RE: Mismatched AHU and CDU for DX system
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Mismatched AHU and CDU for DX system
Ken
Ken
KE5DFR
RE: Mismatched AHU and CDU for DX system
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.
RE: Mismatched AHU and CDU for DX system
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
RE: Mismatched AHU and CDU for DX system
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?
RE: Mismatched AHU and CDU for DX system
HVAC68
RE: Mismatched AHU and CDU for DX system
RE: Mismatched AHU and CDU for DX system
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Mismatched AHU and CDU for DX system
Good luck
RE: Mismatched AHU and CDU for DX system
The above is kinda like your situation where you have a large evaporator and a small condenser. Notice what it says about the humidity