Degree Days - base temp
Degree Days - base temp
(OP)
Hi guys,
I have a basic understanding of degree days and have used them in the past when looking at buildings.
I currently have a process that is heating a product to 30degC. The process is located outside so the heating of same is influenced by external conditions. In this instance I am trying to establish what the correct base temperature should be. Am I correct in assuming that I should be looking at circa 30degC?
Thanks,
Ruby12
I have a basic understanding of degree days and have used them in the past when looking at buildings.
I currently have a process that is heating a product to 30degC. The process is located outside so the heating of same is influenced by external conditions. In this instance I am trying to establish what the correct base temperature should be. Am I correct in assuming that I should be looking at circa 30degC?
Thanks,
Ruby12





RE: Degree Days - base temp
Is that what you need to do?
RE: Degree Days - base temp
RE: Degree Days - base temp
Base temperature for HVAC use of degree days is the outside temperature where no additional heat is needed to maintain the desired temperature inside.
For your process I guess it would be similar. The outside temperature where you no longer need to apply heat to maintain the product at 30C.
RE: Degree Days - base temp
RE: Degree Days - base temp
It's the temperature where you no longer need to apply heat to maintain the product at 30C.
Do a heat balance.
RE: Degree Days - base temp
RE: Degree Days - base temp
Depending on your latitude, radiation may bump your heating degree day (HDD) number up a good bit or down a good bit. Bumps it up at high latitudes (cold night sky outweighs low-altitude daytime sun) and bumps it down at low latitudes (strong, more direct daytime sun outweighs cold night sky).
The difference between the entering and leaving temperature of your process fluid also plays a role. If controlling to 30°C, is the average temperature of the fluid really only 25°C?
Tell us the latitude and the process proportional band, and I'll give you a wild guess. It will be in the neighborhood of 30°C, but I can look at an old program perhaps and tell you how far away it might be...
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Degree Days - base temp
I used HDD to compare the performance of various buildings and see the effect of external temperature and building operation. I went into the analysis thinking I had to find the proper reference point as well, but found in the end that the reference point used for HDD did not change my analysis of how they were performing.
RE: Degree Days - base temp
Let's assume Ruby12's process does have an average temperature of 30°C and ignore radiation and all else.
With a reference of 15°C, a 20°C day will yield zero heating degree days. With a reference of 30°C, the same 20°C day will yield 10 heating degree days.
In the first case, Ruby12's process will need no heating. In the second case, it will need heating. Thus, the insulation project will either have no benefit or it will have a benefit, depending on the reference temperature for degree day calculation. Higher reference temperatures will overstate savings from a heating energy saving project.
Degree day methods are approximations, but you still need to get pretty close to the null point with a reference temperature.
When the subject is a typical commercial or institutional building, the answer works out to be in the 60° to 65°F range so often that you're OK with that range almost every time. You still have to watch out for unusual internal loading and unusual fenestration that can skew the balance point, though.
It's late at night. I hope I didn't get my example backwards.
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Degree Days - base temp
I cannot remember the actual values I would have seen, for an example if I had 3 buildings consuming 10MWh/hdd in Dec, Jan and Feb and then in March 1 of them jumped to 20MWh/hdd while the other 2 remained near 10MWh/hdd, it was a good indicator that some adjustments had to be made.
The more I think of it, the more differences I see with my application and that of the OP's. If I were starting this analysis I would use the average fluid temperature as my basis for HDD.
Late at night here too, will spend some time thinking about this tomorrow and check in on the process.
Cheers
RE: Degree Days - base temp
There are a number of different ways you could approach this analysis. As long as you are consistent and clear you will be alright.
RE: Degree Days - base temp
Of course, there is one other huge factor that we haven't considered. If the savings report is to be presented to non-technical management (imagine a table wrapped in MBAs), keep it very simple. They'll understand if you simply say that the average temperature in the process is XX°C, so that is the base temperature for your calculation, etc...
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Degree Days - base temp
If you are using it for something else, don't be surprised when it doesn't produce anything intelligible.
RE: Degree Days - base temp
I have seen folks use the published data for strange conditions, and the results have been pretty outrageous, as you noted. That didn't stop them from basing financial decisions on the results, though...
Good on ya,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Degree Days - base temp
This is usefull e.g. if you have a heater in you house that runs on oil that is delivered to your house by a truck. The oil company will have a reference and they will know the size of your house. They will then use this to calculate when they need to come around and top of the tank before it runs dry and you start to freeze and you wont have to go gauge your tank once in a while.
If you have gas for heating or central heating or heat using electrical power (or cools) then degree days are not of much use. Some payment systems for central heating may also depend on a degree day like system.
So - a reference temperature is needed as well as a"reference structure" because its not only the outside temperature but also the wind etc taht matters. For a house, the number of occupants would also have to be considered.
In order to show the efficiency of an added insulation you could do a purely theoretical calculation, but if you want to prove it by acutal measurements then you would of course have to compensate for the differences in weater for the two periods - assuming that the operation of the vessels in all other aspects is similar before and after adding insulation.
I dont know if theres a standard practice for this but one simple way could be to get the mean temperaturefor you location for each day in the two periods. Calculate an average temperature and then compensate the difference in the two actual values for heat input for the two periods by the ration of the mean temperature since heat loss is (close to) proportional with dT.
Best regards
Morten