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carlfoster
03-08-2009, 10:24 PM
Sorry, wasn't sure what to title this thread.

I've recently installed an air handling unit and a condensing unit made by Marstair was selected for the match.

The units use R407C and after running it up and adding about 500 grams, I got a full head of liquid going to the TEV, which is a Danfoss TE2. The problem I had then was the suction pressure was too low, it held at about 3 bar with an HP of 16Bar.

I increased the air flow over the evaporator and managed to get the Lp to 4 Bar to stop the icing but it is still to low as I would like to achieve 5Bar suction.

Also, measuring the TD over the coil, I was getting 12 DegC! When you do the calcs, ,this gives you over 5Kw of sensible when the unit is designed for about 4Kw of cooling.

How can this be?! My only theory is that the compressor in the condensing unit is too large for the system and that the match isn't going to work properly.

Any ideas or confirmation of this? If this is so, what criteria other than duty, could I use to match up a condensing unit to the air handler?

Gary
03-08-2009, 10:59 PM
What are the entering and leaving air temps?

Magoo
03-08-2009, 11:31 PM
Hi CarlFoster,
maybe something as simple as the wrong orifice size in the danfoss TE2.
given that HP is high at 16bar and LP low at 3 bar, and coil icing.
magoo

carlfoster
04-08-2009, 06:03 AM
What are the entering and leaving air temps?

When it was 3 bar: Air on = 21 Deg C
Air off = 11 Deg C

After increasing air flow and LP went up to 4 Bar:

Air on = 20 Deg C
Air off = 8 Deg C

The temperature dropped very quickly and the humidity also dropped down from 55% to 48%.

The orifice is a number 2. We chose that orifice size based on the selection software from Danfoss, so I would be surprised if that was wrong. Also, the superheat was about 5 - 6 K so I think the orifice size is fine as the valve was quite steady and not hunting either.

Gary
04-08-2009, 06:17 AM
When it was 3 bar: Air on = 21 Deg C
Air off = 11 Deg C

After increasing air flow and LP went up to 4 Bar:

Air on = 20 Deg C
Air off = 8 Deg C

The temperature dropped very quickly and the humidity also dropped down from 55% to 48%.

The orifice is a number 2. We chose that orifice size based on the selection software from Danfoss, so I would be surprised if that was wrong. Also, the superheat was about 5 - 6 K so I think the orifice size is fine as the valve was quite steady and not hunting either.

So... what's the problem?

BTW, I think your air on/air off temps are wrong. Increasing airflow decreases dT... and I don't believe your coil frosted. 11C does not frost a coil.

nike123
04-08-2009, 07:35 AM
The temperature dropped very quickly and the humidity also dropped down from 55% to 48%.



Drop of humidity, quick fall of temperature and rise on evaporator dT indicate that you are not increased air flow. Instead, that indicate decrease of air flow.

How did you "increased the air flow over the evaporator"?

What is your air handling unit make and model#?

What is your condensing unit make and model#?

hyperion
04-08-2009, 08:54 AM
Depending on the model of Marstair condensing unit, it could have a fan speed controller which would be preset to maintain 15-16 bar without necessarily being fully gas charged. Unlss you have a constant high evaporator load, expecting 5 bar suction could be difficult to achieve, especially as you only have 20degC air on temperature.
Are you sure that your load calculations for the condensing unit are correct?
What is the length of pipe run as if the TEV is functioning correctly and is matched to the evaporator, you should not be getting icing back.
Was this icing right back along the full length of the suction pipe?

tonyelian
05-08-2009, 09:08 PM
hi
first of all u have used a wrong tev it should be tz not te with orifice #2 if u have 5 kw cond unit

BritCit_Juve
21-08-2009, 09:47 PM
Hi
Sorry for the delay
Stupid question but have you removed Marstairs expansion device fron the liquid line?
Also the head pressure controller could be being bypassed which could result in a low evap temperature.
The MCU should be maintaining 280psi head and with a TEV I woul expect approx 60psi on the suction.