TXV vs Electronic Expansion valve

[vijay_study]

Hi,

In one of our projects chillers vendors have quoted for two alternatives for screw chillers.

One comprising of thermostatic expansion valves and one comprising of electronic expansion valves.

Can anybody guide on the pros and cons of both the options.

Regards

Vijay Kumar
HVAC Engineer

[Toolman]

The electronic valve system costs a LOT more initially but probably will make the system cheaper in its running costs . The advantage to the standard TX valve system is its SIMPLE no computer controllers at each valve the old K.I.S.S system.

[WINJA]

The electronic tx valves these days are very good and can run quite a low super heat with less safety margin which makes the evaporator slightly more effecient, a more traditional valve is not necesarily more reliable either I had problems with the sporlan tx valves corodeing on the head and losing its charge which then runs a high super heat gradually getting higher till the chiller trips on low suction preasure and york wanted about $1100 for a new complete valve and wouldnt sell just the element (*******s) .
I think the electronic type keeps better control over wider operating parameters as well ie ambient temp and low and high load

[Kh1971]

HI,

New technology is good but it is costly, so I always want to use the tranditional ways and keep going on the old systems which our service people knows .

May be I am wrong but this is the life

[abet_meneses]

kh 1971
Youre right,in my experience in the past the old txv works effectively whether it maybe on the low,meduim and high temp system,and it is simple in operation and cost less than its counterpart.However electronic txv proves to be the efficient one especially on the low temp system.

[Greengrocer]

TXV's need quite a high pressure differential in order to operate and control properly so must maintain an artificially high condensing temp (more power input). EEV's can operate with a much lower pressure diff = lower power input for same capacity.
EEV's can also acheive closer control via their infinately variable stepper motors. Unfortunately as a new member I cannot paste any tech info into this reply to explain this further.

Ask the supplier to advise how much power input the EEV version will save over the TXV version (difference in EER of each machine). Then look at the difference in cost between the two chillers and calculate how long the extra cost will take to recover with the lower running costs of the EEV model
On a recent job of my own the extra cost of EEV's on 4 off Airedale Air Cooled close control systems (DX) could be paid back in running cost savings in less than 12 months.

[US Iceman]

Or, you could use balanced port TXV's which work quite well and do not require electronic are other gadgets to work.;)

[750 Valve]

an EEV is only as good as the controller running it and still requires fine tuning of the PID loops to achieve the most out of it.

all being equal though (which it rarely is in this game) I think if I was to own a system I'd spec standard TXV's with an electronic EPR stepper on a DX plant (for any circuits higher than system SST) and probably CPC to control it. Electronics are good but after years working on them a standard circuit requires less in service costs over time and with the use of balanced port valves (as iceman mentioned) then the EEV's lose their appeal.

[nike123]

My opinion is, that EEV's mainly have their advantage over balanced port TEV's in applications where there is need for regulation of evaporator capacity by change of superheat. Balanced port TEV have fixed superheat setting, and EEV could change his superheat according to rules in control algorithm.

[US Iceman]

My opinion is, that EEV's mainly have their advantage over balanced port TEV's in applications where there is need for regulation of evaporator capacity by change of superheat. Balanced port TEV have fixed superheat setting, and EEV could change his superheat according to rules in control algorithm.


OK, I'll ask the question that comes to mind.

Why are you regulating the evaporator capacity with a EEV?:confused:

I agree with the red underlined sentence, but I need more explanation from you before I accept the bold print statement.

[nike123]

OK, I'll ask the question that comes to mind.

Why are you regulating the evaporator capacity with a EEV?:confused:

I agree with the red underlined sentence, but I need more explanation from you before I accept the bold print statement.

When you have inverter compressor AC split systems, according to set temperature and measured room temperature (with fuzzy logic), controller regulate compressor speed and capacity of evaporator to match that required in that room. That could be from 30% to 130% capacity of nominal capacity of evaporator. That way, you have continuous work of compressor and precise room temp. regulation in range of 0,5K instead of fixed speed compressor and constant capacity (and superheat) evaporator, where regulation of temperature is in range of 2K.

That is how I think control is done, but I may be wrong!
This is article I interpret like that described above:
http://tinyurl.com/6fcua7
http://tinyurl.com/48ko4k

[US Iceman]

OK, I see where you are going with this now. Since the inverter changes the compressor speed (capacity) a corresponding change in the evaporator capacity is also required (with a constant superheat).

Most TXV's would respond to a higher evaporator load with a subsequent increase in superheat. By using an EEV you could control the superheat at a constant value. The opposite would be true for low loads on an evaporator.

Balanced port valves also have some of this similar ability to modulate capacity. I believe their ability to turn down in capacity while controlling a stable superheat is more precise than a normal TXV though, but not probably to the extent an EEv could.

[SteinarN]

I have installed a couple centralized systems with VFD compressors and Danfoss AKC regulators for room and cases. Those regulators is controlling superheat by pulsing EEV for max capasity. As setpoint is reached, the regulator switch to return air temperature controlling EEV, that is it gradually close the EEV to decrease the evaporator capasity. The compressor maintains constant suction pressure of course. The main point here is to set the suction temperature so high that the lowest capasity evaporator/case just bearly reaches set point. No problem with floating head, all adjustment done from operator panel or PC on site, or alternatively from my office by modem and phone. Its a system I like very much.

[nike123]

I added one more article in my previous post which describes more of function of EEV.