4 ms·
> "I barely know what a BTU is" Neither does anyone else, its one of those archaic units that changes slightly based on who is using it and hangs on in oil and
by voidUpdate 4mo ago
> "I barely know what a BTU is"
Neither does anyone else, its one of those archaic units that changes slightly based on who is using it and hangs on in oil and gas industries, and also air conditioners and heaters.
It was defined as the amount of energy to raise one pound of water by one degree Fahrenheit at one atmosphere of pressure, but that amount of energy depends on the starting temperature of the water, and different things use different starting points, so it ranges from about 1054 to 1059 joules
https://en.wikipedia.org/wiki/British_thermal_unit https://en.wikipedia.org/wiki/British_thermal_unit
- bonsai_spool 4mo agoWhy would the amount of energy required to increase the temperature of a defined mass of water depend on the temperature? This goes against the idea of specific heat (editing to add - I didn't realize that water's specific heat has a temperature dependence and changes around 5% over reasonable indoor temperatures https://www.engineeringtoolbox.com/specific-heat-capacity-water-d_660.html https://www.engineeringtoolbox.com/specific-heat-capacity-wa... )
- actionfromafar 4mo agoPhase change?
- kgwgk 4mo agoNo, it doesn't go against the idea of specific heat. You may be thinking of ideal gases and even then specific heat may or may not be constant: https://en.wikipedia.org/wiki/Perfect_gas https://en.wikipedia.org/wiki/Perfect_gas
- bonsai_spool 4mo agoA specific heat is the amount of energy needed to raise a defined mass of a substance by a defined temperature. What does that have to do with a perfect gas?
- Tade0 4mo agoThe ideal gas model assumes point particles with no interparticle interactions, while all the interesting stuff with regards to all kinds of specific heat happens due to these things in... particular.
- kgwgk 4mo agoThe question was > Why would the amount of energy required to increase the temperature of a defined mass of water depend on the temperature? and the answer is that it does, except if you have a very restrictive model where it doesn't. (Edit: I see there was an edit to the previous comment, I had missed that.)
- bonsai_spool 4mo agoYes, I wrote both comments. Why does water relate to the ideal gas law?
- kgwgk 4mo agoIt doesn't. Your initial assumption that the heat capacity is a thing that doesn't depend on temperature did (because it may apply to ideal gases as opposed to water where it just doesn't stay constant as you noticed later).
- defrost 4mo agoTraditionally it's not a "mass" of water, it's a pound (unqualified) of water. There's an issue right there and one that cracks the door open to "how is pound of water created". Leading into (depending on path to above) the issue of density of water, while famously and often described as incompressible, water reaches max density at 4 C, 3.99 degrees (Kelvin or Celsius) above water's triple point.
- PowerElectronix 4mo agoSpecific heat usually varies with the temperature itself.
- willis936 4mo agoThey're not saying the rate varies but the starting point. Going from 30C to 100C takes less energy than going from 20C to 100C even if specific heat remains physical. Both might get labeled a BTU.
- stavros 4mo agoThey specifically said "one degree Fahrenheit", so the GP's question was "why does going from 30 F to 31 F take a different amount of energy than going from 20 F to 21 F?".
- ChiperSoft 4mo agoWell in the later scenario the water would be frozen and conduct heat within the volume poorly, so applying heat would result in a phase change at the contact point. Latent heat transfer is then going to complicate things.
- stavros 4mo agoIt doesn't matter how well it conducts the heat, since you measure the amount the energy you apply to it (after losses).
- simonebrunozzi 4mo agoSee water's triple point.
- out_of_protocol 4mo agoI blessed to live in a country that uses kilowatts. That works wonderfully (and advertised as such) for AC units
- Ekaros 4mo agoKilowatts as in power consumption or as in refrigeration or heating power? AC is somewhat not straightforward.
- Gigachad 4mo agoIt is pretty simple and standardized. The spec sheet will list the cooling and heating output in kW, and then also the input in kW. The number they have displayed most prominently is the cooling output while the others will be in the specs.
- quickthrowman 4mo agoOne ton (HVAC unit) is 3.5kW of heat moved for each 1kW of input power under relatively ideal conditions, assuming a max COP of 4. The unit chosen to represent the amount of heat that can be transferred is irrelevant, you get the same information.
- dzhiurgis 4mo agoSo it's a gallon of kilowatts?
- anotherhue 4mo agoFour imperial kiloquarts.