3 phase heater

[colonelbogart]

WE have a 3 phase heater used for heating oil for 650kWatts compressor (for ammonia in refrigeration). The heater is 3 phase but the compressor installer wired the heater using single phase splitting one wire to wire all of the 3 phases. and instead of 440V they used 240V...

What will be the general outcome of this?

Thanks

bogart

 

[digitrex]

If the 3 heaters are connected in a Star with a neutral connection available, it is possible to connect single-phase to power up all the 3 heater elements. However the current rating for the supply cable should be correctly sized for the 3 heaters in parallel.

 

[vic3fan]

Digitrex is right. I’d like to add a few considerations:

The way the heater was connected suggests that either the heater has its 3 elements connected in Star or they are left independently and the connection is at installer’s discretion. However, we’ll discuss the Delta connection below.

Let’s assume the following notations for heater terminals:
Hn=the input terminal of the element corresponding to phase n (n=1,2,3)
Xn=the output terminal of the element corresponding to phase n (n=1,2,3).

The rated voltage of 440V means a phase-to-phase voltage in a 3-Phase system, which means further a value of
440/(sqrt3) = 254 V phase-to-neutral.

Now, if we choose the Star connection (X1-X2-X3-N) and the H1-H2-H3-to-phase, it means that all 3 elements have the same voltage applied to them and the current taken from the system is the sum of all 3 currents corresponding to each element. That’s why Digitrex recommended having a proper feeder design.

The 240 V applied to one element will determine a lower current into the element:
I240/I254 = (240V/R)/(254V/R) = 240V/254V=0.944882

The ratio of powers dissipated and transformed into heat is:
P240/P254 = R(I240)2 / R(I254)2 = 0.9448822 = 0.8928

To have the same amount of energy and therefore to have the same thermal effect
E240 = E254 or P240 x t240 = P254 x t254 or
t240 = (P254/P240) x t254 = (1/0.8928) x t254 = 1.12 t254.

This means you have to wait about 1 hour and 7 minutes (t240) to obtain the same amount of heat using the 240V supply comparing to t254=1 hour of using the elements supplied from the 440V 3-phase system. Now it’s you who has to evaluate if this is of critical importance for the refrigeration installation or it doesn’t matter.

If the heater elements are internally connected in Delta (H1-X3, H2-X1, H3-X2) and only 3 terminals are available, this limits our options. We can supply the 3-element heater making the connections
(H1-X3)-(H3-X2)-to-phase, and (H2-X1)-to-neutral.
This means we have only 2 heaters in parallel available, the third one being shortcircuited. This means further an increased delay of obtaining the same thermal effect
tD240 = (1.12 x 3/2) t254 = 1.68 t254, or to wait about 1 hour 30 min to obtain the same amount of heat comparing to 1 hour of using the 440V 3-phase system supply.

 

[itsmoked]

vic3fan; Nicely stated.

 

[rbulsara]

I don't understand why someone have to justify , just because it is technically doable, something which is not right to begin with. You paid for the compressor, you have the right to ask for its installation in accordance with manufacturer's published design/data.

I would ask the installer to correct the installation. Period! It's like you like brown shoes, someone insists on selling you black ones of the same size and quality. The question is why should you accept that if you are paying for it?

 

[itsmoked]

rbulsara; may be they moved it or got it used? Ke never stated the wattage... Maybe it's only a kW or two.

 

[ScottyUK]

440V and 240V both sound like UK voltages. 440V is normally quoted at a main load centre or on the secondary of a distribution transformer. 440V/254V is the absolute top end of the UK voltage tolerance range. Volt-drop in the utility wiring results in the normal industrial voltage of 415V, which equates to the 240V single phase quoted.

I wonder if the nameplate quotes 440V rather than the normal 415V in error? If anyone knows of a location where 440V is a nominal voltage rather than a maximum then please advise - I have a vague recollection that it might be a marine or naval standard.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[itsmoked]

I thought it was common here in the USA about 20 years ago.

220 -> 440

 

[colonelbogart]

its a 2kwatts heater(s) rated 440 V but our system is 480V . our control voltage is 240V 1 phase using trans. the contractor as we discovered lately uses this control voltage instread of wiring an independent one for heaters. as i understand the actual is a series of 1 heater and 2 heaters in parallel making the total thermal capacity to 1.5 of the designed. since the installation of additional wiring/cables for the actual is quite tremendous we are evaluating the actual capacity if we will consider the actual with some minor works. we cannot afford to shutdown to install additional cbs for this heaters (17 in all).

Thanks

bogart

 

[rbulsara]

I would be more worried about the health of the 'control transformer' than that of the heaters. Have you verified the adequacy of capacity of the control transformer?.

This sounds like a messy job, makes me wonder as to what else may be wrong with this installtion.

As for the power consumption (heat output), you need to compare the original diagram with the actual one and find the voltage across each heater element in both cases.

The current thru them will be I=V/R. R= (rated kW/Original rated voltage)
And heat produced will be I^2*R. or V^2/R

Now you can compare the original values and the actual ones.

If the voltage is almost half of the oiginal (240 vs 440), the heat produced will be nearly quarter of the rated kW.

If the actual voltage difference is just 254 vs. 240, it wont make much difference, except that the control transformer may not like it.