K Definition reqd

[sparrow]

The difference between bubble temperature and dew temperature is the temperature glide, which varies depending on the refrigerant, but is usually between 1k and 8k.?

I see this value written for superheat also what are we talking here exactly, with this k symbol?

sparrow

[Quality]

K = Kelvin si unit of measurement for temperature

[TRASH101]

Kelvin (k) is a measurement of the degree of heat present in relation to absolute 0.

Centigrade (c) is a measurement of the degree of heat present in relation to the freezing point of water.

[TRASH101]

The actual period between degree Celsius and degree Kelvin is identical i.e. a temp differential of 8k = 8c

Sorry for the fragmentation Sparrow but on first read I only saw definition of k.

[taz24]

The difference between bubble temperature and dew temperature is the temperature glide, which varies depending on the refrigerant, but is usually between 1k and 8k.?

I see this value written for superheat also what are we talking here exactly, with this k symbol?

sparrow

Amongest other things K denotes differance in temp.

So, air on an evap is 0 deg C air off is -10 deg C the differance is 10K or evap temp is o deg C and suction line temp is 10 deg C = 10K.

K or Kelvin is the SI standard for measuring temp, but we use Celseus or Centigrade. The divisions of K is the same as the divisions of C but C is only a referance to what happens to water at atmospheric pressures.

[sparrow]

OK, now I thought to myself how can we be talking about Kelvin, when people keep refering to 5k superheat etc etc, this is
Particularly off putting when I have had to increase any centigrade, values by 273 to be an equal representation on the Kelvin scale, like if i am transposing ti and t2 they always have to have Kelvin as the temp scale so I add the difference. What am i missing here?

Sparrow

[taz24]

OK, now I thought to myself how can we be talking about Kelvin, when people keep refering to 5k superheat etc etc, this is
Particularly off putting when I have had to increase any centigrade, values by 273 to be an equal representation on the Kelvin scale, like if i am transposing ti and t2 they always have to have Kelvin as the temp scale so I add the difference. What am i missing here?

Sparrow

Nearly there.

Say for example T1 = 0 deg C and T2 = 10 deg C so the differance is 10 Degrees. Differance in temp is 10 deg K.

We use K to stop us getting confused when talking about Delta T (differance in temp), thats all.
Scientists use Kelvin because that is the true measure of temp. We use Centigrade or Celsius because that is the
referance point of water freezing / boiling.

So 0 deg C and 100 deg C would equal +273 and +373 deg K on the Kelvin scale.



taz.

[sparrow]

Kelvin (k) is a measurement of the degree of heat present in relation to absolute 0.

Centigrade (c) is a measurement of the degree of heat present in relation to the freezing point of water.

Say for example T1 = 0 deg C and T2 = 10 deg C so the differance is 10 Degrees. Differance in temp is 10 deg K.

We use K to stop us getting confused when talking about Delta T (differance in temp), thats all.
Scientists use Kelvin because that is the true measure of temp. We use Centigrade or Celsius because that is the
referance point of water freezing / boiling.

So 0 deg C and 100 deg C would equal +273 and +373 deg K on the Kelvin scale.

taz. Thanks for your explinations lads I have it now lol, its the little things sometimes cheers.

sparrow

[Grizzly]

Nearly there.

Say for example T1 = 0 deg C and T2 = 10 deg C so the differance is 10 Degrees. Differance in temp is 10 deg K.

We use K to stop us getting confused when talking about Delta T (differance in temp), thats all.
Scientists use Kelvin because that is the true measure of temp. We use Centigrade or Celsius because that is the
referance point of water freezing / boiling.

So 0 deg C and 100 deg C would equal +273 and +373 deg K on the Kelvin scale.



taz.

Good explaination taz.
You should be a trainer!!!

[taz24]

Good explaination taz.
You should be a trainer!!!

There may be hope for me yet

taz.

[Magoo]

Well done Taz,

Now next lesson for class is LMTD when designing evaporators and condensers or selecting airflow over evaps., relating to room RH. And relating to chilled water HX's, and approach temps., condensers what ever.
This could really solve a lot of issues that guys in the feild get with existing systems that do not perform correctly, usually not designed correctly.
magoo

[Magoo]

OK
LMTD is "log mean temperature difference, "
You can all chime in any time soon with interprutations and application criteria

[mad fridgie]

OK
LMTD is "log mean temperature difference, "
You can all chime in any time soon with interprutations and application criteria

"Grey matter churning", Can only be calculated if using steady state conditions (?)
"churning" need to know temps of all inlets and outlets
Calculate temp differences.
Look on chart for the answer.
I sure it is some think like that.

[desA]

"Grey matter churning", Can only be calculated if using steady state conditions (?)
"churning" need to know temps of all inlets and outlets
Calculate temp differences.
Look on chart for the answer.
I sure it is some think like that.

dTlm can also be used in quasi-steady processes - basically, as long the the temperature changes are not of rapidly-changing nature.

You can use dTlm calculations on a heat-pump component, if you like - it will provide interesting information about the condenser & evaporator.

[TRASH101]

Magoo

LMTD is a derived value to model heat exchange rates for specific values of environmemtal scenarios.

This is all well and good when the heat exchange has a steady state medium (i.e. a fluid whose thermophysical properties will not change drastically through the HX process) and not a refrigerant (or steam) that changes state during this process.

When the medium used can change state then the overall heat transfer coefficient must also change for the proportional volumetric distribution of both states within the physical confines of the HX.

This is the point it then gets confusing. Is this ratio the same as super heat? If so and the mass flow rate remains the same then how can a bifurcation point arise? The mass flow rate must change for that point to exist in a HX with a fixed volume, steady load and stable superheat and the heat transfer coefficient can only change in respect of the pressure/enthalpy and mass flow or am I missing the point.

[desA]

What gets even more tricky with dTlm, is that the original theory does not involve the concept of 'adequate mass charge'.

Mass charge in a RHVAC system can drastically modify the dTlm response of a heat-exchanger, from the assumed design value.

[TRASH101]

the original theory does not involve the concept of 'adequate mass charge'

DesA

please expand on original theory

adequate mass charge implicates the full system rather than HX solely

[desA]

DesA
please expand on original theory
adequate mass charge implicates the full system rather than HX solely

Heat-transfer theory in the derivation of dTlm (log mean temperature difference), makes the implicit assumption that 'enough' liquid/refrigerant/vapour, whatever, is in the heat-exchanger/condenser/evaporator to achieve the required/design value.

In practice, if the incorrect amount of refrigerant is resident in the heat-exchanger during operation, it will actually have a dTlm different to the design (expected) value.

This can be seen using a low refrigeration charge, where a condenser, for instance, is most certainly not flooding, but runs at a very low dTlm value. As the system mass charge is raised, the dTlm of this condenser will begin to rise (push back) up until a certain point, after which flooding does begin to take place, & condenser surface is overtaken by liquid subcooling. At this point, the dTlm appears to begins to reduce again.

So, in the end, RHVAC systems are hugely charge-dependent. We see this a lot in air-to-water heat-pumps, for instance, where the mass charge re-distributes itself during the heat-up cycle. The effect of as little as 10g of additional charge (1-1.5%) can be clearly seen in system performance.