Trane XL19i design question

[jeff520]

I have a question about the design of the Trane XL19i and this is the only resource I am familiar with that could possibly provide some insight as to why Trane decided to design it the way that they did.

My unit is a 3 ton, so it has a 1.5 ton compressor for the first stage and a three ton compressor for the second stage. When the thermostat calls for second stage cooling the condensing unit shuts down the little compressor, waits for a while and then starts up the big compressor.

In the conditioned space this is seen as an increase in airflow at the moment the thermostat makes the call for second stage - an increase in temperature of the conditioned air (because no compressors are running and the evap coil can no longer cool) - and then the temperature of the conditioned air falls to the usual 15 to 20 degree drop from ambient as the big compressor starts up.

I understand the reason for the delay is because it is difficult for the compressor to start against the pressure differences present because the earlier running of a compressor left a high pressure on the condenser side and a lower pressure on the evap side, but the Trane TXV is a non bleed type, so a short delay is not going to balance the pressures enough to make for an easier start.

So, finally, here comes the question:

Since the compressors in an XL19i may have to start against the most difficult set of working pressures possible why not accept the fact and make the following design changes

1/ Replace the 3 ton compressor (in the 3 ton unit) with a 2 ton compressor, change the 1.5 to to 1.0 ton.
2/ Change the logic on the control board to run the unit in three stages. One ton, two ton or three ton depending on which compressors were running.
3/ Allow a compressor to start even if one was already running since there is no benefit to waiting for pressures to equalize if the TXV is no bleed.


An alternative might be to make both compressors the same (1.5 tons for this example) and bring the second one on line (together with the first) for high cooling. The control board could also alternate between the two compressors when only first stage was required so that they each had similar run times over the total life of the unit.


Comments?

[iraqveteran]

Several reasons.....

1. 2 Compressors running would mean 2 seperate circuits. Which means 2 evap coils, 2 cond coils, 2 linesets, etc.etc.. Since your unit has 1 circuit, the compressors would burn in a heartbeat. They would essentially be fighting each other.

2. Multiple circuit units are 3 phase. Yours is single phase.

3. (1) 3 Ton Compressor may only draw 14 amps. Where as a 1.5 ton compressor will draw around 11 amps. (2) compressors running,......11+11=22......Which is 8 amps more than a 3 Ton compressor.......

Which sorta defeats the purpost of HIGH efficiency, don't it?



The system does shut off for a brief minute to allow the system to stabilize before the big compressor comes on. Its a safety feature for the system.




While I commend you for thinking of these things, your kinda on the wrong track. The system was designed that way for several reasons. Most of which I cannot answer.

[Airmechanical]

my am/stand works the same way, however i did modify mine to when high speed kicks in

on the delay my fan reverts to very low speed, i have not had any bad side effects from running it like this

you would need a knowledgable tech to make the modification!



.

[beenthere]

Running 2 different sized compressors at the same time on the same circuit, takes more consideration of oil return.
2 the same size not as much, but has oil return considerations too.

Given that teh 19i comes in HP, or straight A/C, they would have to change things for the HP version to work right.

There is a simple solution to your problem. Your installing company should be able to figure it out.

PS, teh compressor doesn't not always stop as soon as teh electric shuts off, so to prevent teh compressor from being hammered, they give it time to come to a full stop.

[jimj]

In Trane's old xl1600 4 ton Gas/Elect (duel compressor package unit) they used 2 of these.
COMPRESSOR; AP22D-CC1-A, 200-230/60/1, 1.83 TON (1.25-12 SUCTION, 1.00-14 DISCHARGE).

[jeff520]

Beg to differ with you, Iraqveteran but your numbers did not ring true for me so I went out and measured the current at the disconnect box. This current will therefore include whatever the fan takes, as well as the sump heaters if they are on (should not be).

When only the first stage is running the condenser is taking 4.4 amps, when the second stage runs it takes 10.5 amps. I can not measure the power factor, but would assume that it would be similar for both stages.

Allowing 1.25 amps for the low stage fan and 2.5 amps for the high stage fan then the compressor motors would be taking about 3.15 amps for the 1.5 ton and 8 amps for the 3 ton.

Regarding the compressors fighting each other they are connected in parallel, both pulling from the same low pressure pipe and both feeding the same muffler/charge compensator, so I figure it is similar to the two horses pulling my wagon. As long as their two noses point in the same direction I figure they are helping each other and each having an easier time because of it. Three phase has nothing to do with it, and there is no need for separate condenser coils, etc.

Am I way out of line??

[beenthere]

Oil return doesn't always work the way we think it should.

Your 1.5 ton compressor amp drw is for it working against the piping restrictions of a line set sized for 3 tons.
If it was working against the piping restrictions of a line set sized for 1.5 tons, its amp draw would be more.

Currently it is working at a very low compression ration.
2 running at the same time would increase copression ratio, and increase amp draw.

[jeff520]

Understand that, Beenthere, two 1.5 ton compressors working at the same time will take more than twice the power of a single 1.5 ton compressor because of increased losses in the lineset. However, the total current draw will probably be roughly equivalent to a single 3 ton compressor.

The big issue would seem to be the difficulty of starting the second compressor when the system is running and the differential in pressure across the compressor is at its highest. Still, with a non bleed TXV it would seem to me that the problem would exist today every time the second stage started, and it is even more serious because you are trying to start a bigger compressor.

I still think Trane missed an opportunity to not only make the system work better, but also to cut a little cost from the components required to build the unit. Could have been a win/win.

[beenthere]

It probably would cost more to make it work the way you want, not less.

Trane had enough trouble with oil return on them when they first came out the way they are now. Let alone if they would have tried to have both comps running.

[jimj]

Trane had enough trouble with oil return on them when they first came out the way they are now. Let alone if they would have tried to have both comps running.

That is exactly what Trane did on there ycz050

[beenthere]

Theres a couple brands that have 2 comps on 1 circuit that run at the same time in commercial.
Tandems can be a PITA sometimes.

[mchild]

All the major Asian manufacturers have inverter systems that have dual compressors on a single circuit. Typically, they are offered in larger capacities such as 144K BTU, where one compressor is a fixed speed unit and the second is a variable output unit.

They also can gang together multiple outdoor units using the same line set to the indoor units. The controls are highly sophisticated to manage all of it, including oil return issues and load sharing so that not any one compressor is carrying too much of the load.