Warehouse space heating

[Chemister]

I am currently working on an energy management project focused on an industrial warehouse (pulp). I am trying to understand how the warehouse is heated to estimate the energy used and suggest energy saving measures. From what I understand so far, glycol is heated either by a separate boiler (its operated only in winter) using natural gas or by steam from another boiler. The steam exits the boiler header and splits to serve multiple purposes: production processes, electricity generation, and heating glycol (desuperheating steam).

The hot glycol flows through pipes and splits off to unit heaters, where fans move air across the glycol coils to heat the surrounding air in the warehouse. Additionally, the glycol heats air in air handling units, which recirculate the air, and it also heats air from the makeup air unit.

For controlling warehouse temperature, they rely on an On/Off fan operation with the following control logic:

When the fan switch is set to "AUTO," the fan operates only if the warehouse temperature drops below the set point (7°C).
When the fan switch is set to "ON," the fan runs continuously, regardless of the temperature.

I asked people in the facility they told me they have no other controlling method.

For airflow, I asked whether the building has vents to exhaust air. I was told there are no dedicated vents, so the makeup air from the main air handler simply finds its way out of the building naturally. The warehouse walls are concrete, and the door heaters, mounted above the doors, provide adjustable air curtains with 90° nozzles pointing downward to prevent cold air inflow.

My concern: With no dedicated vents and the present control method won’t the continuous heat addition from glycol lead to a gradual temperature increase in the warehouse?

 

[waross]

Look at the heat loss through the walls and roof as a function of temperature difference.
Heat in equals heat out.
When you have solved that, consider heat loss in the air curtains and add that in.
Air curtain loss may be negligible or it may be considerable depending on how often the doors are opened.
And due diligence may suggest checking the temperature of product entering the warehouse.
If the product is above ambient temperature it may be relieving the unit heaters of part of the heating load.

 

[Snickster]

It is not clear what you are saying. So unit heaters are on-off controlled at 7C but glycol continuously circulates through them? Are they individually controlled or all controlled by one thermostat? So make up air unit is always on? Door curtain units are always on? Doors are kept open or closed? Need much more specifics. Why would building heat up uncontrolled?

 

[vint-j]

There are a few things to figure out here. Start with the heat loss through walls and roof—how much depends on insulation and the indoor-outdoor temperature difference. Check the air curtains too; frequent door openings could make heat loss through them significant. Incoming product temp matters as well—if it’s warmer than the room, it might reduce the load on the heaters. How are the unit heaters controlled—individually or together? Does glycol circulate constantly, even when heaters are off? Are the make-up air and door curtain units always running? And how often are doors left open? If the building heats up on its own, there might be unregulated heat sources or something misaligned in the system. Sorting out these details should help pinpoint where things can improve.

 

[Chemister]

Thank you all for the helpful answers. I appreciate your guidance and I will reach out to the facility to gather the specifics you have highlighted. For the calculation of heat flux through construction barriers I will use
Q=A U ΔT
Where A is the area of the barrier (wall or roof)
U- barrier heat transfer coefficient.
U=(1/((1/h1)+∑ (xi/ki) + (1/h2))
h1 and h2 are heat transfer coefficient of the warm side and colder side.
x- thickness of the ith layer
k-thermal conductivity of barrier material.

Should I use an annual average outdoor temperature in my region since the outdoor temperature fluctates?

Unfortunately, I cannot visit the facility to directly measure the norther,eastern, southern, and western walls and the roof temperatures, and the warehouse supervisor provided limited information about the roof. Here's what I currently know:

• Three walls are wood stud with fiberglass insulation and an exterior metal siding.
• The fourth wall is made from concrete blocks.

Since this isn't enough detail to calculate heat loss accurately, what sources would you recommend for assuming the heat transfer resistance (R-values) of the composing materials?

 

[dik]

If the product is above ambient temperature it may be relieving the unit heaters of part of the heating load.

I've been involved with numerous 'cargo' type buildings where the loading docks had infrared heating pipes above the loading doors.

In addition I've been involved in a few buildings with 40' to 80' roof heights, where they have recirculated the heat at the roof to the lower level. It can be 10c at the floor and 40c at the roof level. The climate for these building locations is cold.

 

[Chemister]

It is not clear what you are saying. So unit heaters are on-off controlled at 7C but glycol continuously circulates through them? Are they individually controlled or all controlled by one thermostat? So make up air unit is always on? Door curtain units are always on? Doors are kept open or closed? Need much more specifics. Why would building heat up uncontrolled?

I was able to gather that the unit heaters are always set for 7C with glycol continuosuly circulating through them. The unit heaters are controlled individually by their fans (ON/OFF/Auto mode). The make up air unit are usually always on. The door heaters are only on if the doors are opened in the winter. The doors are always closed during winter and are left open during hot periods in summer.

I was guessing that the warehouse temperature will rise above the setpoint 7C as glycol circulates continuosuly even when fans are turned off heat transfer from the glycol to the surrounding air (via convection) will continue.

 

[waross]

I was guessing that the warehouse temperature will rise above the setpoint 7C as glycol circulates continuosuly even when fans are turned off heat transfer from the glycol to the surrounding air (via convection) will continue.

Probably not.

 

[3DDave]

The transfer from the heaters to the air will be much smaller without the fans. This is the reason there are fans.

 

[waross]

Self correction:
If the airflow from the fans is horizontal there will be little convection.
If the airflow from the fans is vertical, you may have a slight issue on mild days.

 

[Chemister]

I am attempting to account for the energy flows. Air is introduced into the warehouse via the makeup air unit, and there are no dedicated vents, so the air leaks out naturally. At the same time, cold air infiltrates through cracks in the building envelope.I am attempting to account for the following energy flows:
Heat flow from glycol to air = Heat loss through walls and roof + Heat loss due to warm air leaving the warehouse + Heat loss due to infiltration of cold air.

Is it reasonable to assume a mixed air in the warehouse for simplicity starting at the outside temperature and being heated to a final temperature of 7 C (setpoint)?
How can I estimate the infiltration (cold air entering) and extrafiltration (warm air leaving) rates?
Data I plan request from the facility to improve the accuracy of the energy balance:

1. The outlet temperature of the glycol after it has heated the air and gone through a full loop before being reheated by low-pressure steam in the heat exchanger.
2. Glycol concentration as this will help estimate its thermal properties (specific heat, density, etc.).
3. Details on the construction materials of the walls, roof, and floor, including any insulation properties (thermal conductivity, thickness, etc.).

I’d greatly appreciate any guidance to ensure my approach is correct.

 

[waross]

Looking for energy savings?
The quickest and easiest to implement is simply setting the thermostat one degree lower.
Other than that, what goes in must go out;
Makeup air units are often used when air must be exhausted such as when a heater, boiler or other combustion device is exhausting air.
If that is the case, you may consider an air to air heat exchanger on the exhaust of any combustion devices.
Another possible reason for makeup air is to maintain a slight positive pressure to reduce the influx of dust or vapours.
Look at reducing or eliminating the makeup air. Do this very carefully. Unintended consequences may not be a good career building event.

What I would consider if this was my challenge.
Try to determine the heat supplied in to the ware house.
One way is as you suggest, by monitoring the glycol flow and delta T.
Another way may be to measure the weight and temperature of condensate from the glycol heater and combining that with the steam temperature and pressure to find the amount of heat supplied. (A pound of condensate equals a pound of steam.)
Then calculate the anticipated heat loss through the walls and roof.
If these figures show a wide discrepancy, look for missing insulation, water saturated insulation, cracks or openings that allow the escape of heated air.
Also a discrepancy may indicate bad information in.
If, for example you have assumed or been given faulty information concerning R values, your calculated heat loss will be off.
Others have mentioned the temperature difference between the floor and the ceiling.
If you have a high ceiling, the delta T across the roof insulation may be much more than anticipated.

If you do have a large temperature difference between floor and roof, ceiling fans to even out the vertical temperature gradient may be quite productive.
If the warehouse is open to the roof, a false ceiling may be beneficial. Again, beware of unintended consequences. If a new ceiling is damaged by high forklifts, that won't look good on your resume.
The block wall: Has this been insulated by filling with vermiculite insulation or similar? If not, consider this.
If so, a hole in the bottom of a block wall will allow the vermiculite to flow out of the upper cavities all the way to the top.
If the will is insulated with Zonolite brand vermiculite beware of the extreme asbestos hazard of Zonolite.

Possible quick and economical fixes:
Reduce make-up air.
Ceiling fans.
Repair any compromised insulation.
Repair any holes or cracks.
Reduce the temperature set point.
A little more expensive;
Add vermiculite to the concrete block wall.
More expensive.
Increase the R value of the insulation, roof first. Rain and snow may result in greater heat transfer through the roof than air conduction and convection through the walls.
Quick evaluation: does snow build up and remain on the roof in the winter or does snow clear off quicker than on nearby unheated surfaces?.

 

[Snickster]

I am attempting to account for the energy flows. Air is introduced into the warehouse via the makeup air unit, and there are no dedicated vents, so the air leaks out naturally. At the same time, cold air infiltrates through cracks in the building envelope.I am attempting to account for the following energy flows:
Heat flow from glycol to air = Heat loss through walls and roof + Heat loss due to warm air leaving the warehouse + Heat loss due to infiltration of cold air.

The air from the makeup air unit keeps the building pressurized during the winter so that air flows outward only through door cracks, etc. and therefore as long as the MAU is operating there is no infiltration of cold air.

The heat flow from the glycol to the air is based on the capacity of the unit heaters. I believe you are concerned when the unit heaters fans are off and you have natural convection. I don't think you can quantify exactly how much heat is loss while continuously circulation when the thermostat is above the 7C setpoint and fans are off. I don't think it would be enough heat to significantly raise the temperature in the warehouse. This is because the design temperature must have been 7C for sizing the unit heaters and determining how many needed. Therefore, the heaters should be just able to maintain the inside temperature at 7C at lowest winter temperatures outside. So at design 7C heat output of heaters equals heat flow out through building walls/envelope. So consider this the design calculated heat flow out through the walls, building envelop at 7C when the heaters are operating with fans on which is the maximum heat input into the building space possible. If the temperature rises above 7C the heater are off, but now there is even more heat transfer to the outside at a higher inside temperature than 7C than even what all the heaters were designed for with fans operating. Since the heaters under convection flow can only output a fraction of the heat as with fan running they will never equal the heat output through the building envelope when the temperature rises above 7C. The only way the temperature can rise inside the building is if the output of the heaters is greater than the heat flow out through the building envelope.

That being said I would put the glycol recirculation pumps on a thermostat control such that they only circulate glycol when temperature outside is say 9C. Not only are you wasting energy through convection at the unit heaters but you are wasting a lot of pumping energy.

Heat flow from glycol to air = Heat loss through walls and roof + Heat loss due to warm air leaving the warehouse + Heat loss due to infiltration of cold air.

Heat loss out through the building envelope can only be reduced by adding more insulation. The make up air unit must operate to provide ventilation air or provide for some make up air requirements for any process equipment. You cannot get rid of this. This air is the only air that exits the building. Theoretically there is no other added load from the MAU other than the heat you put into the heating coils in the unit to keep it at a maximum supply air temperature of 7C. As this air passes to the warehouse and out the cracks theoretically it should not pick up any heat from the unit heaters if the unit heaters are set at 7C also and keep the inside space at 7C approximately.

Usually an HVAC engineer will perform an energy analysis on a building using computer programs and established energy conservation methods. Seems like you are trying to wing it without much experience.

 

[Chemister]

Thank you so much for your help and insights. I couldn’t have better understood the system without your guidance.

Yes, I definitely lack experience and knowledge of HVAC systems, as this is part of a university project for a course I’m taking. I understand that the warehouse is pressurized by the makeup air unit to prevent the inflow of unconditioned outside air and that it operates continuously. However, I have some questions about scenarios where the outside temperature exceeds 7°C, which is often the case from June to October.

During those months, should we reset the setpoint temperature of the system? I understand that the makeup air unit should remain operational to prevent the influx of dust. However, I don’t see the need for heating the makeup air with glycol during this period, as heating demands, unlike production demands, are flexible and vary throughout the year. Wouldn’t it make sense to turn off the heating system entirely during the summer months?

That said, I’m unsure what to do with the low-pressure steam if it’s not utilized for glycol heating. My understanding is that we cannot recycle the steam back to the turbine, as it could condense into liquid somewhere along the turbine line. This could cause issues, as turbines are designed to run on gas.

If we keep glycol heating during the summer, the makeup air could get unnecessarily heated above the outside air temperature, potentially reducing thermal comfort inside the warehouse.

 

[waross]

The air make-up unit appears to be sized for 68,000 cubic meters per hour.
It appears to have thermostatic control that will close the valve on the glycol circulation when the ambient(?) temperature is above 7 degrees C, leaving the unit supplying unheated air. You may wish to verify that that system is still operational.
68,000 cubic meters in equals 68,000 cubic meters of heated air lost per hour.
I would determine the actual delivery rate of the make-up unit. The flow may already be restricted.
I would also attempt to determine the pressure difference between the warehouse and the exterior.
I would convert the actual delivery volume of the make-up air unit to air changes per hour.
With that information to hand, the question is;
Will a lower pressure and fewer air changes per hour be acceptable?
The air flow through the make-up unit may be restricted to reduce the changes per hour.

That said, I’m unsure what to do with the low-pressure steam if it’s not utilized for glycol heating.

Not your problem. Close the steam control valve. (This may be done automatically already)
If you use less steam, the de-superheater will produce less low pressure steam.
This will reduce the steam demand on the boiler and the boiler fuel feed rate will automatically be reduced to compensate.

 

[Snickster]

You can split the of the entire warehouse HVAC systems controls for hot summer months versus cold winter months at a given outdoor air set temperature. Attached is a specification I prepared for generator room heating and ventilation controls which does this.

 

[Snickster]

The attached provides information on determining building envelope heat loss.