Tag: sensible heat

Every piece of air conditioning equipment is capable of moving a certain amount of heat BTUs (British Thermal Units) at set conditions. In most cases during the cooling mode, a portion of those BTUs will go toward changing the temperature of the air and a part will go towards changing vapor water in the air into water that collects on the evaporator and then drains out.

The BTUs that go towards changing the TEMPERATURE of the air are called SENSIBLE and the ones that go toward removing water from the air are called LATENT. The percentage of the capacity that goes toward sensible cooling at a given set of conditions for a given piece of equipment or space is called SENSIBLE HEAT RATIO (SHR). So a system that has an SHR 0f 0.70 and 30,000 Total BTUs of capacity at a set of conditions would produce 21,000 BTUs of sensible cooling and 9,000 BTUs of latent removal because 30,000 x 0.7 = 21,000 and the rest 30,000 x 0.3 = 9,000.

Higher SHR (closer to 1.0) = More change in temperature and less humidity removed

Lower SHR = less change in temperature and more humidity removed

In the HVAC industry, there is a set of standard conditions used to compare one piece of equipment to another. When a system has an SHR rating listed it would often be at AHRI conditions unless the specs state otherwise.

When doing a load calculation a good designer will calculate and consider the internal and external latent and sensible loads and match up with equipment accordingly based not only on one set of design conditions but on the range of seasonal and occupant conditions that the structure is likely to experience based on the use, design and climate. By following ACCA (Manual J & S) and ASHRAE (62.2 & 62.1 for example) standards a designer will have guidelines to follow and this includes matching the space SHR to a piece of equipment that will make a good match at similar conditions. It does often need some digging into manufactures specs to interpret this data for the equipment.

In the example above from a Lennox unit, you can see that the SHR is listed and highly variable based on outdoor temperature, air flow setting as well as indoor wet bulb and dry bulb temperatures. In this example, you would need to multiply the total capacity x SHR to calculate the actual sensible and latent capacity.

This example from Carrier has no SHR listed, instead, it lists the specific sensible and total capacities. You can easily calculate the SHR by dividing the sensible capacity by the total capacity and the latent is simply the sensible subtracted from the total.

The cool thing is that this understanding can help both designers and commissioning technicians to match equipment properly and even make further adjustments using airflow to get a near perfect match which leads to lower power consumption, less short cycling and better humidity control.

— Bryan

Here is another great explanation from Michael Housh from Housh Home Energy in Ohio.Thanks Michael!


I’m going to layout and compare the Sensible Heat Rate equations for both the air-side and water-side of HVAC, to help draw similarities and dive deeper into the science behind these equations.  This is the beginning of a series to try and help us all gain a deeper knowledge of where these equations come from. The more we learn about the two, the more similarities can be drawn between them. This will hopefully allow a technician to be more comfortable when faced with different systems in the field.  I should also note that while the equations can be complex, they are a great reference for those who would like to build them into spreadsheets (or other formats).

 

Sensible Heat Rate Equations:

 

Air

Water
Q = 1.08 * CFM * TQ = 500 * GPM * T
Where:Where:
Q = sensible heat transferred (Btu/hr)Q = sensible heat transferred (Btu/hr)
CFM = quantity of air (ft3/min)GPM = quantity of water (gallons/min)
T= dry bulb temperature difference (°F)T= dry bulb temperature difference (°F)

 

The only thing I will say about the Delta-T side is that it is the measurement of dry-bulb temperature, this is something I think all technicians know and have a decent grasp on.

 

Like most things in our industry these are “rules of thumb” equations, however, both derive from the same lower level equation, which is as follows:

 

Q = M * C * T

 

Where:

Q = sensible heat transferred (Btu/hr)

M = mass of the fluid (lb/ft3)

C = specific heat of the fluid (Btu/lb)

T= dry bulb temperature difference (°F)

 

I’ve often heard Bryan say that air-conditioning is about moving pounds of refrigerant.   We move pounds of refrigerant to create air-conditioning, and we move pounds of air to deliver that air-conditioning to the space.  As you may have gathered from the above equation the Sensible Heat Rate is derived from moving pounds of a substance (in our case air or water).

 

I’m not going to dive into the details of the above equation at this time, but wanted to share where both of these equations stem from. What I’d like to breakdown in this article is a deeper understanding of where the 1.08 (air) and 500 (water) constants come from.   Both CFM and GPM are actually what provides the “pounds” of the fluid (air is a fluid), based on density and specific heat.

 

Density is defined as its mass per unit of volume (or weight per unit of volume), and specific heat is the rate at which an object will give off or absorb thermal energy.  Both density and specific heat are moving targets, but in the “rule of thumb” below are the values that are used.

 

Density (lb/ft3)Specific Heat (Btu/lb)
Air @ 70°F & at sea-level (14.7 psia)0.0750.24
Water @ 60°F62.371.0

 

I’m going to solve for the water-side first.  We have to take into account that our measurement for water is Gallons Per Minute, so for anyone who doesn’t know, there are approximately 7.48 gallons in 1 cubic foot.  Using the density from the table above we can solve for the weight of one gallon of water 62.37 / 7.48 = 8.34 lb @ 60°F.  Since our end result of the Sensible Heat Rate equation is Btu/hour we have to convert our GPM -> GPH (gallons per hour).  So, our 500 is a simplification of the following values:

 

Constant = 8.34 (lbs/gal) * 60 (min) * 1 (specific heat) = 500

 

When we provide the GPM in the Sensible Heat Rate equation for water, we have already accounted for its density (mass), specific heat, and converted to gallons per hour.

 

Next, let’s look at the air-side.  Here we have to account (just like in the water-side), that our measurement is in Cubic Feet per Minute, and since we are solving for Btu/hour we will have to convert CFM – > CFH (cubic feet per hour), we also have to use the density to account for the mass of air that we are moving, and the specific heat.  So, our 1.08 is a simplification of the following values:

 

Constant = .075 (density [ lbs/ft3]) * 60 (min) * 0.24 (specific heat) = 1.08

 

So, just like the water side, when we provide CFM to the Sensible Heat Rate equation, we have already accounted for its density (mass), specific heat, and converted to cubic feet per hour.

 

I hope I haven’t utterly confused you on such a technical topic, but stay tuned for more in the series to help bring the Sensible Heat Rate equation (and the air / water side) closer together.

— Michael Housh

This tip was a COMMENT on the sensible heat ratio tip left by Jim Bergmann. As usual Jim makes a great point, once you get the “sensible” capacity for a piece of equipment at a set of conditions you can easily calculate a true target Delta T.


Another interesting thing you can do with this information is to determine the approximate target temperature split under any load condition. There are some additional footnotes on that chart likely saying the return air conditions are at 80 degrees at each of the respective wet-bulb temperatures.

To do so, find the sensible capacity at any set of conditions, for example at 95 degrees outdoor air and 1400 CFM, the sensible capacity is:

At 72 wb 25,010 BTUH

At 67 wb 31,730 BTUH

At 63 wb 37,360 BTUH

At 57 wb 37,930 BTUH

Using the sensible heat formula, BTUH = 1.08 x CFM x Delta T

Delta T = BTUH /(1.08 x CFM)

So…..

Delta T = 25, 010/(1.08 x 1400)

or 16.6°

Delta T = 31,370/(1.08 x 1400)

or 20.74°

Delta T = 37,360/(1.08 x 1400)

or 24.70°

Delta T = 37,930/(1.08 x 1400)

or 25.08°

So you can see also that the target temperature split has a lot also to do with the return air and outdoor air conditions and it has a lot of variation

— Jim Bergmann w/ MeasureQuick

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